Hm, if they're taking lots of terminators then it might be best to just take as many Boyz as you can and swamp them with lots of attacks. Powerfists will destroy your nobs with instant death. Though, you only have 20 boyz.. best bet I'd say is to still swarm them and take advantage of their initiative 1 powerfists and hope they roll a 1 (which is likely, if you can get odds of 5-6 boyz against one terminator).
Lootas might be a good idea as they can sit quite far out of range of the terminators and force them to roll lots of saves, so they're definitely an option worth considering.
If there's one thing I've learned, it's to never underestimate ork shooting. I've lost a fair few games because of the sheer number of shots being put out by orks.