She is two sizes smaller so her grapple is pretty useless, per my modified rules, she won't even guarantee disadvantage except for her, but I might relax that because if she can flank then it should count. She doesn't stop or restrict him because she is "tiny" not "small" and he is "normal"
Do your homework Kashtanistan. It looks like Toll of the Dead is the best bet for damage and because of the known wisdom of the Bugbear Commander. If there's any way you can improve your chances, do that too. There's time to cast Bane on him, but don't get hit in that round or you will lose your concentration.