I don't know much about multiboards (due to the fact that I've never use one in a map) but maybe I can help you out.
After making your multiboard, make an Real variable (ex. "BossLife") and set it to 100.00. Then when the boss is attacked, set BossLife to the boss's health as a percent (using triggers). Also in this trigger, after setting the variable, change the label of the item in the multiboard (not sure if you can do this) to BossLife as a substring (using conversion in the trigger) + substring "%".
If this doesn't work, use a Leaderboard instead. I'm positive you can change labels in those, just use the same steps.