Actually, as a total sidenote, I've found that grinding AI3 with:
Mark the same as your towers
14 towers
6 dragons
6 other efficiently-costed attackers
4 Shards of Gratitude
is really, really good. The SoGs not only save some games that you would otherwise have lost, but they keep your average winnings in the 17+ electrum per game range, which is way better than the 12 you'd get otherwise -- and the time taken to win the games is maybe a few seconds longer, so the overall time efficiency is actually better than without them.
I've tested it with Life, Water, and Entropy, and they all work well that way.