If my opponent plays a {witchbane orb} and I play {bonfire of the damned}, I know he wouldn't take damage but would his creatures? If he can't be targeted, can his creatures be targeted through bonfire that targets him and his creatures?
I am pretty sure the spell would fizzle as it would not have a valid target for the player portion
Opponent isn't a legal target. You can't target the player, so the spell would sit in your hand until you could play it. The spell doesn't fizzle.
So no damage delt to creatures?
You have to have a legal target to cast it, {bonfire of the damned} targets a player and then in the aftermath hits that player's creatures. There is no target creatures anywhere on that card.
Ruling to support this?
Orb says you can't be targeted. Spell says target player. Can't target the player who controls the orb so there's no legal target for the bonfire. Gotta get rid of the orb before you can use the bonfire.