Don't put a point in zeal. Rely on your merc to do most of the killing in those tight areas. Support the merc with holy bolt too. That really helps to keep them alive in those situations where you can't kill something, be it magic immunes or tight areas like the maggot lair. It will take time to get through those areas, but you will get through them. Another way to support the merc is to buff him with enchant using the demon limb weapon (if you have it). That adds to their attack rating and gives little bit of fire damage too.
Also, use your other auras as necessary to help the merc out...salvation, cleansing, etc. I used those a lot as a hammerdin to keep the merc alive when I couldn't really contribute to damaging enemies. For the most part though, you only have to worry about this in a few places. Most of the game you can spam hammers and clear the map of mobs with ease.
