[Manzcar]

Before talking to master sean the second time (after you have the dungeon key) take off your weapon and several pieces of your best armor, but leave your inventory window open.

The reason is that your shadow is a carbon copy of you when you enter. Therfore your shadow will have no weapon and less armor. This means less attach strength and less defense.

After spawning into the first dungeon where your shadow is quickly put on your weapon and armor.

The marlone fighter and archer are actually hiding in the dark room right before the final room with your shadow. You are able to kill them there instead of when you are fighting your shadow, which makes concentrating on your shadow that much easier.

Again before you step into the final room to meet your shadow take off your weapon and best armor. Same reason as above.

I was able to complete the lvl 20 test using this strategy and lots of pots (I kept hitting the pots and stones right after they cooled just to be sure I didn't die)

And that's my two cents