All my techs are 10. With any attack hero lvl 20 and up I use 350 ballista and 399 transport with no losses, ever. The only losses I've got since maxing techs are on lvl 4 npc's with lower level hero's, so I up ballista from 200 to 250. By the way, this is the best page I found on the subject:

http://www.theevonywiki.com/NPC

I've used this from lvl 3-5 and it never failed me.