Hey, if a Pokemon with full health, uses Endeavor on a Pokemon that also has full health, but has more total health than the Pokemon using Endeavor, will the Pokemon with more health have his health taken down to the other Pokemon's total HP? Or will it just say: "But it failed!"
Yes it will. If Swellow had 100 Hp and Blissey had 700 HP, Swellow's Endeavor would bring Blissey's health down to 100.
The other Pokemon with full health's HP will get taken down to the user of Endeavor.