I think Strength should be your main attribute which effects damage output and reduces AP cost.
1. The act of polymorphing should cost life. Lore reasons could be that polymorphing isn't magic but the act of forcefully changing your body through brute strength. Kind of like how the werewolves from the Netflix series "Hemlock Grove" change. They reform their bones and "shed" their human skin.
2. Constitution would be your second primary stat which reduces the life cost of polymorphing.
For example,
Form of the Wasp - Become an agile wasp that can shoot poisonous stingers.
- Lose 40% health. (Health cost reduced by 2% for each point of Constitution)
If you had 10 Constitution it would be "Lose 20% health."
- Increases Dexterity by 60% of your strength.
If you had 10 Strength it would increase your dexterity by 6 while in Wasp form.
