Fathom should focus on making surfaces slippery with water or mucus as a counter for wall jumping and sliding( or viscous punch placements). Make enemies slide or end up in locations way further than they were expecting. Throw off a rotating lash in the air with powers he doesn't understand from the deep.
Like Blue Gel but you can half paint it around corners to slide someone into a team fight without counter spell ? Slide a cubed viscous with a heavy punch ?