If you don't have flamers, see if you can proxy flamers.
This is assuming he has a lot of guardsmen. Like an assault squad with 3 flamers (Sergeant+2 others) can jump in and with a good position burn down an entire squad killing on 3+. The bolt pistols fired by the rest of the squad will enable probably the entire squad to have flamer wounds allocated. Don't forget if a flamer overlaps another flamer you count those wounds twice.
Then if that wasn't enough they can charge in.
If terminators could deep strike to a decent position to not have every
IG focus on them they can charge in and cause some damage.
I am assuming massed infantry for some reason.
We need to know what he has