One of my experimental deck to test which pokemon partners well with Passimian's ability. The first thing that came into mind was to pair it up with one of the bulkiest Pokemon in the game, the Mythical Island Golem. When equipped with cape, even the strongest attack such as Charizard's Ex Crimson Storm, Golem can tank one hit from it (of course assuming Golem had attacked before hand).

The game play is to start off with Passimian; dealing as much damage to the opponent's pokemon while tanking the early game hits from them. Typically, you'll have time to attach at least one Energy to Golem before Passimian is knocked out. Once that happens, you can transfer all Energy to Golem and begin applying serious pressure using Guard Press, which not only hits hard but further enhances Golem’s durability.

Pokemon Communication and Iono will help us build our Golem as soon as possible here. The Giant Cape and Lillie will ensure maximum durability and survivalbility on Golem, and Cyrus will pull out any dying Pokemon.

One of the weakness here is Sabrina, thus why I decided to add 2 Leaf to counter it. The best set up is to have Passimian in the active spot, and have only ONE Golem in bench. Passimian is meant to be taken out in the mid-game giving the opponent 2 points. It wouldn't be ideal to have another Golem in bench because then it would be prone to Sabrina.