Getting good grips from smashed top half

My passing style is body lock from the butterfly ---> smashed top half guard. When I get to smashed top half I usually have really low underhooks. I try to transition my grips to higher underhooks or a crossface underhook combo. Usually my partner gives me a hard time by keeping his elbows toght and blocking my bicep when I go to reach for the crossface so I just end up either muscling through it or losing the position. Is there any technique I can use to get better upper body grips from there?