How can I kill the last green and black dragons in one turn? Otherwise, the remaining one will take out one elf.
And secondly, will the dragons eventually start attacking each other when there are only a few left, enough to kill each other?
Here's a specific example: on auto-battle, the Towers killed almost all the green dragons, and even though the green dragon only has 4 HP left, the black dragons don't attack the sniper. Also, the HP of the last black dragon is reduced to a minimum, and both are killed by the Towers: one at the end of the round, the other at the beginning of the next. In fact, it's not even necessary to use auto-battle. You can reduce the HP of both dragons to 5-10 and kill them in one round. Artillery is overpowered.

https://www.youtube.com/watch?v=bTA0Ysy_haE