How can I use *War Club* with a unit target?

I have a beefy unit that I want to have a war club type ability. I want him to go up to a FRIENDLY UNIT, kill it, and gain a really strong attack for ten attacks. Simply changeing the target from tree to allied or ground or any combo of thoes doesn't work. Do I need to trigger it? If I do, how?

I think warclub itself is mountain giant only.

If you want, the spell could be based of death coil with a range of maybe 50 [basically touch them], and then have the trigger increase the damage of the unit for 10 attacks...

You'd have to record each time the unit attacks though...