You actually have to spend an action to activate trace.
Ok Poliwag, you're the fastest here, so try to sleep Monohm with Hypnosis before he does something annoying. Horsea, you target him too but with the offensive edge of an Ice Beam.
For the second action, I'd like Kappa to launche a Signal Beam at the Ralts in the canopy while Poliwag gives it (Ralts) a nice little Encore to reward it for its fantastic teleporting abilities.
Then, Horsea use Ice Beam on Monohm again while Poliwag strikes him with Dig. The ground's a little mucky and underwater, but as a water-type this shouldn't present a problem.