The random attack idea actually works much better with the Dark Whisper ability used by Tiamats (the most advanced Hydra class). I think you need Monster Skill to give them this ability but it's a random breath attack in a cross shaped grid. The thing is, the panels only appear on tiles of the same elevation, so if you position it over uneven terrain, you can force all the random hits to appear in a single panel. It works best in places like Deep Dungeon.

I'm not sure if Truth and Untruth can work that way.

On a side note, Tiamats are the best breedable monsters in the game!