In general, Borderlands 2 respects players who are steady, focused, and thorough. Unless you're a Gunzerker, over-aggression usually leads to being surrounded and fighting for your life, and passing up crates merely means not having enough ammo or cash to purchase stronger weapons. Tips #1-5 is mainly a recap for Borderlands veterans but are essential for new players to read, while tips #6-10 generally pertain to elements specific to this sequel.
1. Take advantage of the four-player cooperative multiplayer.
Borderlands 2 is centrally designed for multiplayer, with vehicles that can hold more than one player and loot drops that improve the more people are in your party. Sure, enemies also become more dangerous, but the benefit of having a team is immense, if just for having someone who has your back when you're out of health and fighting for your life on the ground. Not only does the meter fall more slowly, but your partner can take care of surrounding enemies before reviving you so that you won't face as much retaliation when you recover.
But really, the game is hardly as fun playing solo. The five different classes are designed to work in tandem, with the Gunzerker as the damage-dealing tank, the Siren as the elementalist, the Assassin as a sniper and duelist, and the Commando as an all-around powerhouse. The Mechromancer hasn't been detailed in full yet, but she will likely fit in as a keep-away character who can summon robots to distract enemies from the main party. Also, the new trading interface and character mods that affect the entire team both naturally lend themselves to multiplayer. Just make sure you join a game with people you know or trust, or else it might devolve into four players only looking out for themselves, which defeats the whole purpose of multiplayer…
2. Get your fundamentals down.
After becoming accustomed to the first-person shooter movement, it's important to note spots for cover. Though there is no cover system per se, finding a wall to hide behind and reload is just as important as aiming for headshots to land critical hits. If you don't have anywhere for cover, circle strafing around an enemy and moving along the perimeter of an arena (as in a boss battle) are viable options to ensure that multiple enemies can't surround you easily.
That said, also look at the mini-map frequently for enemies that may not be easily seen so that you aren't caught flat-footed. Some enemies can spawn out of the blue, so always be ready for a retreat if you feel surrounded or for a firefight if you see a suspicious-looking door in the distance.
Pay close attention to the cooldown for your special ability, which for most classes is a life-preserving skill. Of course these special abilities have their offensive uses as well, so don't be afraid to lay the smackdown with them. Be on the lookout for support skills and characters mods that reduce the cooldown for your special ability.
3. Elemental weapons rock!
Understanding enemy elemental weaknesses is key to surviving the wasteland. Specific enemies can sometimes only be damaged significantly by choosing the right element, especially when compared to the first Borderlands where enemies don't frequently have elemental weaknesses.
Fire inflicts continuous burn damage and works well against human enemies, while lightning inflicts continuous shock damage and short-circuits robots. Armored enemies tend to be weak to corrosive weapons and strong to mostly everything else. The explosive element works more like an area of effect attribute where aiming becomes less important and a narrow hallway becomes an enemy deathtrap. One of the perks on inflicting elemental damage is that an enemy can't hide, because damage numbers continually pop out, revealing the enemy's location. Better yet, it can even save time and precious ammo when you know an enemy will die soon without any need of an extra shot.
4. Don't forget about the slag element.
The new elemental type introduced in this installment is a purple substance known as slag, which inflicts more of a multiplier status effect than straight-up damage. A non-slag weapon inflicts about thirty to fifty percent more damage on a slagged enemy, so this element can't be ignored (especially when you've been slagged yourself).
Flying solo, inflicting slag is a tad difficult since you have to interchange weapons (and aim correctly) for the slag element to take effect. So it may not be worth your time, but in case you decide otherwise, choosing a weapon with a fast fire rate and weapon swap speed like an SMG or pistol will give you more time to deal damage. As you might expect, slag is much more useful in teams, where one character with a lot of firepower (likely the Gunzerker or the Commando's turret) can inflict slag on the crowd of enemies for the rest of the party to take advantage of.
5. Carry an assortment of elemental guns and a slew of different gun types.
Naturally following from the last two tips, you should look toward amassing a variety of fire, shock, corrosive, explosive, and slag weapons. Having the right elemental gun in a fight works wonders, as is switching out to the optimal elemental types before a fight. As you progress through the story, the number of weapons you can swap between increases from two to four. Along the way, it will be good for memorization to slot a specific direction on the D-pad with a specific element.
That said, you should also organize guns by their type as well. Obviously, sniper rifles and launchers are solid long-range weapons while pistols and shotguns are solid short-range ones. But the reason for carrying different gun types has as much to do with range as it does with ammo. In Borderlands 2, stronger weapons tend to consume multiple bullets at a time per shot, so running low on ammo can become a frequent problem, particularly when you want to go into a boss battle with your primary weapons.
Meanwhile, running back to an ammunition dispenser can be a hassle, so having a backup weapon that consumes a different ammo type is a worthy strategy. Having a shotgun is a recommendable secondary weapon for boss battles, since they tend to take place within confined arenas and get dish out larger chunks of damage in case the fight gets up close and personal.
6. Leave no stone, pile, box, or crate unturned.
In the first Borderlands, picking at every loot pile was more a force of habit than a good use of time. Most piles spewed at best a few dollars and a common gun. However, in Borderlands 2, being thorough has a much more tangible reward.
Apart from obtaining extra ammo, Eridium can occasionally be found in everything from loot piles, small boxes, and even washing machines within Crimson Raiders HQ. Eridium allows you to purchase important upgrades to your backpack capacity, bank capacity, and maximum ammo capacity for every gun type, particularly rockets which characters start off carrying only six. Sometimes enemies will drop Eridium, but I would say that at least 50% of all the Eridium I found came from examining every loot possibility. This means a lot more room to carry additional weapons for overall firepower and loot storage, as well as a lot more rockets against bosses and badass-type enemies.
7. Locate Michael Mamaril in Sanctuary as often as you can.
Most NPC interactions only amount to a few humorous lines of dialogue, but Michael Mamaril is one NPC that you definitely want to speak with. He is a fellow Vault Hunter who apparently travels throughout the wasteland killing skags and gathering loot that he's willing to share. Every time you meet him, he will appreciatively hand over a weapon and it's normally a green, blue, or even purple gun.
So as long as you can find him in Sanctuary, you'll get a free bonus weapon. Michael changes his location often, but he normally hangs out in key locations like Crimson Raiders HQ, Scooter's Garage, Zed's Clinic, Moxxi's Bar, Marcus Munitions, etc. Every time to visit Sanctuary to restock and sell your excess wares, make sure to search for him.
8. Spin Moxxi's slot machine a few times.
Two slot machines sit next to the left of Moxxi in her bar. The cost for one spin depends on your overall level, but it's usually low enough to pop about a thousand dollars and see if you have any luck. Most of the time, you'll receive a gun that's at least worth the same amount as the cost for a spin. So whenever you travel back to Sanctuary, give it a whirl.
Now, I haven't gotten anything particularly good at all from the slot machine besides some customization skins, which is why I'm emphasizing the words "a few times". That said, the slot machine eventually spins three bandit heads (which throws a grenade at you so back away quickly!), so when that happens just step away from the machine and go on your merry way. Nothing stops gambling addiction like a slot machine that can kill you.
9. Use vehicle for infinite ammo and for clearing entranceways.
Now that vehicles have regenerating health, their infinite ammo and powerful rocket launchers are an incredible asset to clearing out any huge monsters or at least chipping away at the monster before leaping out and finishing the job. The only downside is that the experience for killing enemies in a vehicle are significantly reduced. Due to the sheer power of a vehicle, most dungeon areas have a gate of some sort that stops its advance. But that doesn't mean you can't remain at the gate, pull some enemies out, and give a taste of some vehicular manslaughter.
10. Saving and quitting can be the fastest shortcut and is the best way to restore crates.
Sometimes, the quickest way to reach a fast travel station is simply saving and quitting the game. In particular, some side quests can end with you extremely far away from a fast travel station, and since enemies respawn quickly, the last thing you may want to do is kill more enemies on your way back and find loot that you can't fit in your backpack.
Quitting and then re-entering the game also has the important effect of restoring all the item boxes, which don't respawn much more slowly than enemies. This means that if you take on a side quest that takes place in an area you've recently completed, you won't have as much extra ammo from crates or fresh treasure crates to loot.
Do you have a tip you want to share? Place it in the comments and, if it's good, it will be placed in this guide.