It depends on which neutral creatures we're talking about. You're unlikely to defeat Coral Priests.

1) Your army must include guards. They absorb some of the damage from adjacent units, reducing the damage taken by your main attacking stacks.
2) All empty stacks should be filled with single units of guards. In battle, these single units either protect archers and novices (if you're playing against melee units) or distract ranged units by moving within one turn's range. Or they draw melee units away. These are sacrifices, in any case.
3) If you're fighting against ranged units, place the stack that they're targeting under regeneration. If you're fighting against melee units, healing might be more effective. Against ranged units with physical resistance, Stone Skin will help.
4) Use your racial ability wisely.

Oh, and yes... play as a mage. Tactics and aggression are essential. Healing or regeneration depending on the situation, depending on who you're eliminating. And, of course, prioritize learning the first spell as your first upgrade.