You don't have to kill Myrkul's sending now. Just explore the rest of the dungeon until you gained some levels and found enough magical weapons to hurt it.
Mage scrolls are better used for learning spells, not for casting them directly. Right click the scroll and scribe it, once you learned it you can select to memorize it when sleeping (similar like the druid) so you can cast it the next day.
I agree with AvatarOfLight, it might be better starting over with a better balanced party. If you add a multiclass thief to your 3 rangers, 1 druid and 1 mage you have covered all needs (a cleric or druid for priest spells, a mage or bard for arcane spells, a thief for picking locks and disarming traps and some fighting power), but I recommend reading the manual first and deciding what party you want to play afterwards. If there isn't a manual in .pdf form on your CD you can download the official manual at
Sorcerer's Place - Icewind Dale Tips, Tricks & Hints .