Step 1) grab fleece that gives you 10x DMG but 1 heart
Step 2) go to the purgatory
Step 3) select fleece and fight it till it becomes just all 4 bishops
Step 4) do the run again
Step 5) get lucky and get dagger as weapon
Step 6) use dagger's powerful attack to nearly 1 shot the bosses
Step 7) win and get the achievement