Yea,imps were a real pain for me too,but this is wat i did most of the time,I smashed,charged Ice,N+1 while it was stunned then defense,N+2,then it will mostlikely die.
Multi aggro rooms,yeah this was a pain too,but the best thing u can do is get in a corner and hope u dont get multi aggroed,btw,wait for the !! isntead of just attacking random monsters.
PS:Dont worry,it will get easier along the way.