Man, everyone has this skill.
Every time I went out with my warrior to cap another elite, this was the one the boss had.
In Thunderhead Keep, the boss locations differ, so he could be anywhere.
There is also a boss in Iron Mines (near the bottom, on the ice next to the bonus, where the siege engines are bombarding you) that has Cleave.

I hope you are only going for this skill because you are a completist. It is not the most useless elite in the game, I leave that epithet for Dwarven Battle Stance, whose description goes a little something like this.

"All of the successful strikes (which is unlikely since you will be blinded/guardianed/aegised/riposted/deadly riposted/etc) of the slowest attack weapon in the game will cause interrupts if they hit during the use of a skill.
P.S you can't speed up your attack, or use an anti evasion skill, because this will cause your stance to end."