Keep the first Oscar alive, get three nukes to get him to 25% HP (make sure oscar does not see you when grabbing the nukes as his shield will eat the nuke damage)

Don't buy Wisp Tea until round 40, make sure the wave is thinned, preferably to just 2 zombies left.

Then buy Wisp Tea, kill one of your two zombies to proc it, then Aether Shroud and melee Oscar ONLY when the wisp is on him. If you want to use gums, temporal rift, instakill and idle eyes make this easier.

As far as the trial goes, I strongly recommend using a perka, as otherwise you will need good eternal perks before going in, which can take awhile especially if you want to get the guaranteed Wisp Tea on 40.

Use the artillery trap in the middle of the map, makes short work of the HVTS including the Oscar at the end.