If you can disco your pet and cast poison on it, that works great, just cast often enough that it's always trying to cure itself, and that gives you resist gains at the same time. I also got good gains by discoing the pup and letting it fight a rotting corpse.
Otherwise rotting corpses all the way. I used 2 at a time for a bit, just cast the odd heal on your pup if his health gets too low.
Or just use your pup as he is. I haven't noticed any decent improvement considering the time it takes to finish a pup's healing, so I'm not convinced it's really worth the trouble
Wenchy