Fire anyone?

Give anyone who can't have a Burna a combi-scorcha, have a big mek with burna, have lots of small-ish boyz mobs with 3 burnas in each, maybe some buggies with scorchas as well?

You'd want to avoid stuff that can't take many special weapons like trukk boys, and just fill all available spots with fire.

Loot a Hellhound, doesn't get hotter than that...

and skorcha's are nice and you can make quite a number of nasty looking flame equipped vehicles.
Also lot's of flames on the vehicles as well.

Combination of everyone above. Loot a hellhound and give everyone that can some kind of flame weapon, burnas for all da boyz and scorchas on every vehicle.