is this a good idea?

i was thinking of using something like this:
me: mind blast spammer (don't have build yet)
2 searing flames heroes (don't have build yet)
and 1 other hero
and then i use arcane mimicry

Im afraid Searing Flames Eles are a little bit too old school now. There are other elites which work a lot better, plus, with 3 eles everything is focussed on firepower whereas in hard mode it is often useful to load up a bit on defense.

Mind blast hero is a good idea, but id say replace the SFs.

If you simply HAVE to use SF, then stick a paragon in the other slot and have him use [card]"They're On Fire!"[/card].

if you are interested and want to use maybe SF?

then 2 SF heroes and 1 ToF para tank...works wonders mass burning so mass damage reduction plus supporting chants from para make life a lot easier..used this build to complete Raisu Palace HM with me as ranger using h/h! works wonders even in annoying missions like that!
.SF is not needed just need a few skill like mark of rodgorts and some nuking spells to pretty much the same effect...if you wish to use MB then just make sure to kit out your heroes with energy runes so they can spam MB without ever losing energy :P

peace all

edit: mind blast pwns all in terms of fire magic skills IMO...SH close second - love AoE more than anything