The achievement is to have completed the mission "A Dam Fine Rescue."
*This is an unmissable achievement
SPOILERS!* You've been warned ;}
It should be the 6th story mission. You'll get the achievement after you defeated the W4R-D3N in the BloodShot Stronghold. You will rescue Roland right after and turn in the story quest and the achievement will unlock.
Some strategies I used for playing this mission were to be a bit higher level or the same level as W4R-D3N he's Lv. 13. Go for W4R-D3N’s eye you'll get “CRITICAL” there. Single player won't be too hard to beat just use some kind of Rockets and or save grenades or find some. Make sure you equip the best rockets you can get from either play the Slot machines in the Sanctuary or buy them as well as grenades. Rockets will really help you.
If you are playing co-op with your friends make sure you heal each other and try to call out if you’re down. Co-Op with 4 people is a bit overwhelming for this mission and may surprise your skills. Just try to stay alive and shoot and help your team mates out best you can.
Maya / Siren – Best character in my opinion. She is good for lifting Robots in the air so they don’t smash you and she’s just an all around badass. She can heal team mates with gun damage, she can also lift team mates up from far away when they go down.
Zer0 – is good for stealthiness; he's got a cloak. Sneaking around enemies and also healing your team mates when they are down.
Axton – is good for setting up his turret and killing leftover enemies or taking down some shields.
Salvador – is good for going around shooting things with duel weapons but be carefully he can go down pretty fast and could be a risk to the team if he’s always out there dying. He reminds me of Krogans in Mass Effect. So try to stay a little back so you can help the rest of the team out. He's very good with damage since he's got the Gunzerker thing going on which makes him do a lot of damage to Raid Bosses.