i've got a good tip! use your enemies for your advantage! if there is a big group of hostile units on the battlefield, there is a chance that most enemies will attack it, defending themselves, meaning they will also defend you.
I found a glitch, when you have the plastic shield, and the Evade-Every-Hit-As-dashing gadget, If you deal the first damage in an arena, then the enemy hits the plastic shield, you will evade EVERY other hit, UNLESS you dash..
if you are having problems because of the reloader, switch to another weapon, go back to the weapon that you had the problem with, and TA-DA! also Level 11 challange is possible! You just have to make sure that the STRONGEST enemies hit you, they will die quickest...i don't think i even got the other ones to die..ever..