Even under modern Struggle mechanics, it's possible to outrace the recoil.
If two players each have a Slakoth with minimum Attack and maximum Defense, both holding Leftovers, they can Baton Pass Ingrain and Aqua Ring to the Slakoths. After running out of PP, Slakoth will start using Struggle, but that only deals 17-21 damage, plus 80 to itself in recoil (a critical can deal up to 31, damage, making the maximum total hit to either side in a turn add up to 111). Meanwhile, end-turn effects will have it gain back 20 from Leftovers, 20 from Ingrain, and 20 from Aqua Ring, for a total of 60. Then the following turn, Slakoth can't use Struggle because it will be Truant instead, but it still gets another dose of the end-turn effects to gain back another 60--that's 120 in all, more than enough to restore it to full health even if it took a max roll critical.
Since neither Slakoth can switch out because of Ingrain, once they get to this point, the battle is guaranteed to go on with no way of ending unless one player forfeits, disconnects, or runs out of time.