I used to play Archon: The Light and the Dark on my Commodore 64 for hours against the computer when I was a kid. As an adult, I keep having ambitions to re- (or de-)make this game despite the numerous official remakes out there.
For some reason, I chose to use 12x12 sprites for this implementation.
One problem is the old Phoenix vs. Shapeshifter fight. Not only is the Shapeshifter a high-HP character, but this fight devolves to "who used Immolation slightly first" slugfest.
Possible solutions:
Another is aura-based attacks in general; they're all centered around the piece that triggered them. The banshee has a cooldown-based AoE, and phoenix has triggered (+cooldown) AoE.
Of the projectile-based creatures, the differentiating factors are: velocity and damage. Velocity can be problematic because of the ease of dodging. Movement speed of pieces should be a consideration for balance, because troll vs. unicorn is another huge issue.
Tiers: high-powered pieces like dragon and unicorn move quickly and fire quickly; medium-tier pieces like basilisk and valkyrie move and fire medium-ly; low-tier have large tradoffs like manticore moving slowly with medium-speed quills, and archer doing the same.