In a game I'm running at the moment, I've got two characters capable of casting wither (very low level death spell) and at least one death shard, possibly two. This exploit works off three things.

1. Wither, even cast by the same caster, stacks.

2. Wither affects every enemy target on the battlefield

3. Once a character's stats are too far into the negative, they seem to die automatically. This is likely due to their constitution being so low their hitpoints effectively become zero.

When going into a battle against a large number of low level troops (which the computer seems to quite like), casting wither (supported by death shard(s)) four times is sufficient to outright kill all basic soldiers on the field. With two casters alternating this is finished before they even reach melee range.

As you can guess, this means most fights are currently trivial. While enemy champions and large monsters are not a concern, this is enough to effectively defang any attack from an AI army.