Enemy
Slow but sturdy like most Traitless Wave Attackers, Mr. Mer has above-average range, deals decent damage, and has a 100% chance to create a Lv.3 Mini-Wave.
At 100% strength magnification, his Mini-Waves deal 3,200 damage.
Strategy
Use Wave Shields if possible, otherwise you'll need tanky meatshields and Wave-Immune Units to take him out. Since it is difficult to hold him back with power alone, certain units, notably Macho Crystal Cat, can help a lot against him. Beware of his range; while not overly high, it's more than enough for him to hit some commonplace units such as Cameraman Cat, Octopus Cat and Awakened Bahamut.