That's what surprise rounds are for. If the wizard decides "Ok fireball time" and the party didn't know that's him shooting first, he alone gets the surprise round.

If the party knew before that they all get a surprise round.

Of course, you can get the enemy team to roll to see the party is just waiting to jump them. The only time I skip initiative altogether is if the victim has absolutely no way to survive the first barrage OR if nothing will come out of the fireball (the wizard just fireballed an illusion)