You consider immunity by removing the attack value from the Wizard when calculating the battle.
From the rulebook:
Immune to (Something): Monsters may have a trait that makes them immune to something, be it spells, blunt weapons, Physical Attack, or heroes of level 2 with a Strength of 4 or less on a Tuesday. If a monster is immune to something, then all of that thing’s Attack Values (that are not directly applied to other specific heroes) are reduced to zero. Other traits or abilities of that something cannot target or affect this monster. Noninteractive effects and abilities, such as Light or keywords, are not affected by this trait.
The Ghul card has the trait “Immune to Fighters and edged weapons”.
Heroes who are Fighters have their Attack Values reduced to zero while fighting a Ghul. If a Fighter had a trait or ability that helped non-Fighter heroes in some way, those could still affect the fight.
Likewise, if you have something giving the Wizard a bonus, that bonus (and the rest of the Wizard's attack) would be reduced to zero. This would have no effect on any other heroes given that bonus unless they were also Wizards.