In this video guide I will show you how to defeat Oliver Collins in the ruined city underground. Now this is the first boss related achievement in the game and you won't miss it. I mainly used the Caster class's ability called blazing roar
. With that on cool down I would wait for him to attack and move in for a couple hits and back out. Rinse and repeat with the same few moves will make this fight fast. Just keep hitting him when the blazing roar once its off cool down. And use it as soon as its available. Don't fret too much if you die. Your ally will revive you a few times unless he is dead.
Another thing I found out you can do is get the boss's aggro and let your ally attack
him until he changes on him. Once he does go in for a few hits of your own. Mix this up with what I said above and you should have no problem. The achievement will pop once the cut scene play for a bit.