for heavy, tomislav and stock is the best to destroy waves like robo demo or robo soldier spam
also get a medic to pocket you, or upgrade your heavy stats to max.
if your team struggling with a giant overhealed robo heavy or robo soldier, use the brass beast and get a kritzrieg medic ubering you
i dont really play scout but, stock and mad milk combo is really good, dont have much to say
for engie, use the jag and resque ranger, use the rescue ranger to safe your sentry from far away with it and jag lets you build buildings faster,
put your dispenser near the power classes like demo heavy or soldier so they can always gets ammo and health
for medic, ALWAYS upgrade your overheal and uber charging rate to maximal first, it can help you revive dead teammates faster and gives charge your uber faster too
use stock if theres only mini robots spam, but use the kritzrieg if your team is fighting a giant robot