With Trevor you need 2 whips. We will use A and B to denote them. With whip A equipped jump then press attack ([]) then press final attack (O). as Trevor begins to go upwards pause the game within 4 frames. Equip whip B, without unpausing equip whip A. Unpause the game. Immediately hit [] then press O. Pause the game. Repeat these steps to get "infinite" height. Trevor will go slightly forward and up each time. Eventually, you will get over the high invisible walls as long as there is no ceiling (most rooms have ceilings).