My main strategy of trying to encounter Twisted Squirm is to play as Rodger, equip Duck Toy (or whateva it's called) and clown horn, join a solo run and wait till floor one, look around the map, and if there's is no twisted Squirm or you find him and get his research, leave and repeat. For some reason, rates have their highest chances of spawning between floors 0-4, so this has been working well for me. I currently have 98% on him.