1. Cull the Weak can be cast multiple times (on the same unit)

Duplication Steps:

1. Enter a battle with a trained unit that has low HP (E.G 2/36 HP)
2. Heal the unit (or let it regenerate)
3. Use cull the weak on the unit... 

Results:

• CtW will ONLY subtract the HP that the unit started combat with (2HP).
• The unit will not be killed and can be culled over and over again, provided you heal it as needed.

I went from 60 to 1300 mana by culling one pioneer multiple times in one single battle. Youtube Footage:

2. Cull the Weak has an incorrect description.

The description states that it heals 40 HP and restores 40 mana.

In practice, it restores 40 mana and has no impact on HP.

3. Cull the Weak can be used on untrained units.

Cull the Weak can be used on all the following unit types:

• Summons (Shadow Wargs and the like)
• Reward Units (Spearmen / Archers)
• Goodie Units (Sand Golem's, Dogs, Wolves)
• City Militia (This can net a huge amount of mana per defense, with no penalty)

It seems abusive that you can use it on summons and city militia as there is no penalty to doing that.