Big orge lady have very very high strength. You have to weaken her or make yourself stronger (by magic and potions).
Can't say much about not able to grapple, haven't seen situation where was no reason to not be able to. You can run out if stamina, then you have to hold in position while your stamina regenerating (thats endurance dependant). When you're injured you have less endurance, hence faster you loose your stamina.