Finished my mono Psychic type run today and I've gotta say, Malamar saves the day (no pun intended). The inverse field really messes with most of your enemies because the AI can't adjust to the inverted type scaling and it's easy to obtain. If you don't want to train any new Pkm I advise you to utilize Roserade's Grassy Terrain. This should greatly increase your damage output since half of your Pkm deal special damage and Mist increases Sp. Def of Fairy type Pkm by 50%.