For this achievement you must defeat Lester at the top of Castle Prevant.
The battle is fairly simple as you don't really need to participate much.
Do not attack Lester at all, let Sigmund and Edward handle him, while you and Aya take out any minions that are around
or if you have leveled up decently and have plenty of healing items go ahead and help out Sigmund, if you defeat Lester within 2 minutes you will get a situation bonus.
After Lester is defeated Sigmund will cut the chain and net you the achievement as well.