I picked up Borderlands: Game of the Year Edition earlier this summer. For a mere $30, I got the full game plus all four DLC packs. (The DLC is on Marketplace for $10 apiece, and the game is also available for digital download for $19.99. Once again, physical discs at retail prices win out over digital downloads.)
Borderlands is one of those games that's much better played in multiplayer. Although I got it nearly three years late, the announcement of Borderlands 2's pending fall release got people excited enough to revisit this old game over the summer. In the very few sessions I managed to play co-op, I managed to get all of the multiplayer achievements just by playing the game, really only "boosting" the duel-related ones. (I discovered later, though, it wouldn't have mattered so much, since these achievements are obtainable in split-screen co-op with an offline account.) For a game that is so much better in multiplayer, it does seem strange there aren't more co-op achievements; but, so be it.
There are a lot of annoying achievements in the game, mostly due to the repetitive task quests. The "Braaaaaaaaaaaaains!" achievement is a good example. To get it, you have to complete all of the quests from TK Baha in the "Zombie Island of Dr. Ned" DLC area, which amounts to collecting zombie brains by killing zombies with headshots. You first have to collect 10, then 25, then 50, then 100, then 250 brains (by headshotting zombies). If you activate this quest first, it can be easier (since you can just collect brains along your way to complete other quests), except you have to keep coming back to TK Baha to turn in the brains every so often (since you can only collect brains when you've started the quest — any extra brains you may collect will not count towards the next quest). I did the achievement last, so I had absolutely nothing to do but find and shoot zombies.
The most annoying achievements, though, are the collectible achievements in the "Claptrap's New Robot Revolution" DLC pack. These achievements require you to collect a certain number of items, which appear from Claptraps when you kill them. There is no shortage of Claptraps, so that's not the problem. The problem is the fact that these items do not always drop. They are relatively rare, and what item you may get is entirely random. Collecting 20 oil cans isn't very hard, except when you take into account that you have maybe a 1 in 5 chance (an optimistic estimate) of getting a special item dropped from a Claptrap, and a 1 in 6 chance of that item actually being an oil can, as opposed to a pair of pink panties, a fish in a bag, a slice of pizza, a Claptrap bobble-head, or a pair of 3D glasses. (In my experience, the 3D glasses were the most common — annoying because they're also what you need the least amount of — but I can't tell for sure if that was fact or just dumb luck with the virtual dice.)
The common boosting method is to get to the end game (spoiler alert), where the Mega Interplanetary Ninja Assault Claptrap rolls around the screen, shooting you the whole time, dropping "Divine Wind" Claptraps when it stops. The deal with the Divine Wind Claptraps is they will eventually self-destruct (right next to you, if they get their way), so you don't have to shoot them to kill them. You can hide in certain spots where none of the Claptraps can get you, but since there is really no penalty for dying on this mission, that's really optional. Just leave the game for several hours, come back, finish off the MINAC, and pick up all the items dropped by the suicidal Claptraps that are filling up the screen. Even then, there are reports that you may have to do this more than once just to get all the special items.
There is a trick that helps it go a little faster. If you shoot the Claptraps as the Mega Claptrap drops them, it will keep dropping them to maintain two Claptraps on the screen at a time for the several seconds it is stopped. I happened upon a machine gun that had corrosive and electric damage that I could fire a few bursts and kill several Claptraps at a time. When I ran out of ammo, I opened chests to find more, and failing that, I switched over to my shotgun to kill the Claptraps as they emerged (keeping a careful eye on my ammo count so I had enough shots to finish off the MINAC). I could get up to around 16 Claptraps for every time the MINAC paused.
It also seemed to me that, if I left the items on the ground, there was a greater chance of getting special items compared to if I picked up the common items before moving on. Also, by leaving the special items I didn't need (like the 3D glasses, which I collected before I even got to the endgame), the special items I didn't have would be more likely to drop. As if the game, when it decided to drop a special item, took note of what items were on the field at the time and favored items that weren't already there. Again, I don't know if this is actually the case; it may have just been my need to find some kind of order to the chaos. The fact that I was getting rather tired of picking up every item (which you have to do by making sure your crosshairs are centered on an item before you even think about pressing the button) probably contributed to me wanting to find an excuse to leave the items on the ground.
Even with the insane number of Claptraps destroyed (I managed to level up from level 61 to 67 in the process), it still took me two separate 2-hour sessions of mindless Claptrap-killing and item-collecting before I finally picked up the last item I needed.
After that, the last few achievements were easy. Game complete.