I think the percentage rule might make sense. If you do less than one damage, take the fraction of damage that would be inflicted and make it a percentage chance of doing one damage. So if your attack would only do .1 damage, you have a 10% chance of inflicting damage. Perhaps, as a compromise, you can cap this fraction damage so you always have at least a 10% chance of causing damage (or maybe 25%). 10 units won't kill an enemy automatically, but it'll at least damage it.
This.
l8r)