Amdusias is the first really nasty boss, and everything after him will be level 16 or higher, so I would go do more quests until you're closer to level 15 - the skills you get there will really help - Healing and your own Explosive Arrow.

If you've built your ranger with a focus on dexterity and ranger skills, switching to warhammers will probably not be all that effective unless your bow is underpowered AND you've got a bunch of points into Strength. Getting close enough to Amdusias to use the hammers without taking an exploding arrow to the face will likely be difficult anyway. If you can get that close and have points into Splitting Arrow, you could try that.

I think your wisest course would be to come back at level 15.