There are no mage spells required to take out the captain, though another couple levels probably wouldn't hurt. You could clear out anything remaining on Sentinel Island and in Broken Valley before continuing (there is one quest/location in each place that you must wait until after you have the battle tower to do).
Do you have any temporary boost potions (strength, if you are using normal weapons, etc)? That may help you do damage faster.
Do you have any spare skill points you could put into your choice of weapon expertise or Bleed, etc?
Do you have your creature summoned to help? In D2:ED I found it was much more helpful with a ranger head than warrior, and best with a mage head.
Do you have the best enchantments available on your weapon(s) and armour that you have the formula and ingredients for?
If you have not been to all the merchants since the last time you leveled, you could go check for equipment upgrades.
You can also head to the fjords to level, from the path in the far south west of Broken Valley. There is a barrier preventing access to the full fjords, but between the mountain path and a couple beaches you should be able to get another level or two (not sure; the enemy placement was changed in DKS). There is also a mine in the starting section, but it may be too much above your level to clear out now.
Welcome to the forum.
