Otherwise, you can use MB to stun/convert them to get a bit of time. Stunned monsters also can't attack your merc (not even Oblivion Knights.. A stunned oblivknight can't cast IM can he ) which increases his life expectancy quite a bit.
My Lightning Sin just uses light with no fire at all, so yes i either hope there are some non light immunes around to kill first to get bodies for death sentry, before blastin those immunes to bits, or just wait for my shadow, and merc to kill a couple, to get those bodies for death sentry. Which is sometimes takes a while.
I cant comment on how useful the 20 fire blast would be, however the 20 in shock web for the synergy is very much needed i think in the overall build of the trapper.
So overall if you have a bit of wealth and can afford good gear for your merc to stay alive and kill those pesky monsters, hold off on the 20 in fire and beef up your shadow master, while putting the synergy in lightning with shockweb, you could always go 10 in fire to help your merc out also if u really find it slow with the immunes, all the plus skill equip u can equip can really beef up that skill anyways.
Good luck hope it helps