Odin, use Poison Point, but keep it disabled for now.
Start off with a super-effective Sludge Bomb. If possible, use the toxic goo to absorb some of the Leech Seed. If this isn't possible, just try to do as much damage as possible.
Next, while Chikorita is busy hiding behind a Protect, use Focus Energy.
Finally, Double Kick Chikorita, and try to knock it into the poisoned spot.
According to the Data Audit Thread, Focus Energy should cause multi-hit moves to crit.