In Gen 7, there are 4 critical hit ratios:
- +0, with a 1/24 chance
- +1, with a 1/8 chance
- +2, with a 1/2 chance
- +3, with a 100% chance
Spacial Rend already has a high-crit ratio, raising it to +1. To get it to +3, you can either use Z-Sleep Talk, give it a Lansat Berry and wait for Palkia’s health to get low enough for it to use it, or use another Pokémon to Baton Pass a boost from Focus Energy.