As far as I know, resulting damage is based on team's overall Weapon Damage.

When I see mage LB on WP last pack, for example, the damage is much higher in relic party than in party with one 50lvl guy.

And if I'm not wrong, it doesnt matter who will use LB - if it's based on team, then best way to use LB with 2 monks is to leave LB for monk with lowest DPS, because anotther one will deal more damage while weaker will be casting LB.