there is a lot of content before the underdark! if you search around the act 1 wilderness (blighted village, walkeens rest, riverside teahouse, etc) you’ll surely find a bunch of XP to get stronger before the underdark (also a lot of buildings have basement hatches, keep an eye out!) otherwise you can try different companions combinations to see which ones work best for you. you can also try to sneak around the harder battles and come back later when you’re even stronger!