A lot of wizards are inherently imbalanced in WFRP from a combat perspective. This is supposed to be counterbalanced by Tzeentch's curse, but in my experience, that seems to trigger less often than it should. Even an Apprentice wizard can hit harder and more accurrately than a fighter of equal experience, thanks to their magic missle attacks, and it only gets worse when they become Journeymen. I am familiar with this problem, though my party usually doesn't focus on tactical combat. I know of no "simple" Slaughter-Margin like solutions, because the imbalance in combat capabilities is big and I presume you still want the other characters to stay relevant. My solutions, some aspects tried and some theoretical:
Wizards can cause damage easily, but the enemies should know that, or find it out early in the fight. So they concentrate on the wizards. After the first or second crossbow bolt most wizards are barely standing on 1-2 Wounds, which is a very good incentive for the rest of the party to defend them, and for them to frantically try to stay alive. A few goblins with bows pose minimal threat to any fighter in medium armor, but they can knock a wizard down a few pegs in a single sneak attack salvo. So enemies being reasonable and not stupid, and trying to take out major threats can do wonders for combat encounters. I used this several times with a Journeyman Light wizard and his insane Eye-laser.
Wizards are supposed to be feared and hated in most of the Old World, so play with that. Have allies or bystanders around, or simply set up circumstances where blatant and flashy magic use would cause problems. This incentivizes planning on part of the players and makes them think twice about using magic to solve all problems. Because if you are allied with Sigmarite zealots while fighting orcs, subtle magics might remain unnoticed, but a burning missle of death would cause questions, which could lead to burning death on part of the caster. I also did tries out this one, the end result being the party slaughtering both the attacking goblins and the caravan they wanted to save. It was pretty interesting combat- and roleplayingwise as well.
Hordes of mooks
Most wizards don't get real AOE attacks until later careers, and while mooks pose only moderate threat to a well-equipped character, most wizards would have trouble with a lot of zombies/skeletons coming their way. A single one, or even half a dozen pose no real threat to a wizard, but a dozen or two would pose enough of a threat. The important part is that they are unable to blast enough of them in time, so they need tactics and strategy to win. Hordes of mooks are one of the things that armored fighters with Parry and Dodge can handle much easier than wizards, at least in my experience.
A boss with adds
A wizard might be able to cause a great deal of damage, but anything that can take it due to armor and high Toughness (Rat-ogre, Minotaur, etc) can stall them for several rounds. If said boss has minions of their own, especially ones who are smart, combat gets interesting again, as the wizards have to choose between blasting and trying to survive, while the others have to balance themselves between defending the wizards and killing things. I sort of tried this, though the wizard opted to go full blasting and ended up burning a Fate point.
What not to try
Wizards vs wizards. Because rocket tag is not fun or tactical, and the winner usually comes down to luck. It is very much in-line with WFRPs spirit, but it doesn't make for interesting or tactical combat.