In my current PT, I finished Act 1 and decided to stay at LVL 6 during act 2 to see how far I could get. But it turns out to be psychologically annoying because you can't see the XP amount anymore as it is hidden by the "level up" flag. So to add some relieve I'm just leveling up Wyll. I don't want him in my party as a combatant anyway. I use him only as my charismatic trader. So that allows me at least to check and reasure myself that I'm still gaining XP.
It looks like it's not going to work. I managed to clear a few levels in Moonrise, did the House of Healing and other shadow encounters succsessfully, but I get stuck in the defense of Halsin's portal. At lvl 6 I don't have the spell power to keep the archers from destroying the portal. (Have Shadowheart, Gale, Lae'zel and Astarion) I do have one ice storm scroll, and combined with lightning glyph of warding does quite some damage on the incoming crowd, Keeping melee fighters away seems feasible, but I need more big area spells to keep the archers away and kill them quickly.