let them attack - we'll resurrect if the enemy is winged and strong...
the mana vortex allows you to worry less about mana, unlike other towns - that's why the Dungeon is so strong-)
my standard army for clearing on week 4 consists of:
troglodytes (upgraded) - 100 units, harpies/witches - 40-60, beholders (can even be non-upgraded, but that's worse) 30-40 ... and that's it! Hero Jedditt, I take tactics if the opportunity arises and definitely level up earth magic! I leave the town with doubled mana, possessing 1st and 2nd level magic. Just as battles are fought for skeletons when playing Necromancers, I don't skip any battles for experience when playing Dungeon - it's very important to get expert earth magic as soon as possible...
troglodytes cover the beholders and witches - with the tactics skill there are almost no problems; if anyone reaches the shield of own flesh - they get a nasty surprise. Slow, Haste or even Bless - something will be found in the guild, and that's already a serious help given the relative abundance of mana