Alright lets talk about Squirtle's Withdraw first off it is light armor, I'm 99.9% sure you can smash it with Sheik's F-Air but B-Air might be safer for off stage play. Mewtwo's Confusion can snag Squirtle right out of it depending on how good you are I like to punish Squirtles that go low on recovery with a b-reversed Confusion into D-Air. Ike can literally crush Squirtle's Withdraw with any aerial without having to trade due to his massive disjoint. For Zelda make use of Din's Fire to place the flame a bit lower than the ledge to stop either Withdraw or Waterfall and punish with either a Lightning Kick or D-Air. For Wolf just ledge grab and drop below with B-Air or D-Air when Squirtle transitions out of Withdraw.
A good method for punishing Squirtle's recovery is simply edgehogging to bait out Waterfall then roll onto stage and punish the fact that Squirtle didn't sweetspot the ledge with an Up-Smash or Up-Tilt to string into a combo.
Squirtle's low traction causes players to routinely overshoot Waterfall and rely on the fact they can slide back off stage to grab ledge afterwards and that's a huge punish window.
-Omni
