try to practice countering mutas and guards in the... expansion mission, terran, #3. computer will send you mutas and guards over and over, not too much but enough to bother you.
if they send overlords, just keep 12 wraiths patroling around, if see any incoming forces, first kill the overlord, cloak, then guards, then gather your ground forces, like marines, stim pack, you will have plenty time to finish off the mutas. Never counter mutas with wraiths, wraith does explosive damage, that means, only 25%, which is "5", to mutas, but marine does at least 6. goliath does only 25% to mutas too, 7 damage, but they are too expensive.
I would use wraiths to patrol and as supportive force. marine is always number one.