OOC: The only restriction I can see to making an AoO is being flat-footed, so Filch will be able to make it. Filch gets to use improved grab only if he hits with two claw attacks, so he still cannot stop the plant with his AoO. Filch's AoO cannot penetrate the plant's bark (rolled 19-1=18 for 1st attack while levitating).
I had forgotten that Filch cannot take any actions until his next turn due to the dimension door (I remembered he could not take any actions the same turn), so he could not activate mental barrier when the plant fired the spikes. As a result, he plant hit one more time for 3 points of piercing and 4 points of electrical damage. Filch has 22 HP remaining.
Filch hits with two claws (worst hitting roll 22-1=21 for L, best missing roll 12-1=11), and grabs on (15 vs. 14 grapple, thanks to negative levels). Filch inflicts 7 points of damage to the plant, but in the process, Filch gets shocked for 4 points of electrical damage from the plant's body.
Both combatants disappear from view for 7 minutes, only to reappear in exactly the same spot. The plant is not dazed (rolled 19-2-17 for NL Will save).
Round 436: The plant tries to break free, but cannot either try. The plant has 28 HP remaining. Filch has 18 HP remaining. Filch is next, and both combatants are in 100N90E35Z. If the plant could speak, it would say, "Let go of me if you know what's good for you."