Well, considering Allosaurs are CR 2, and have a +6 to hit, yeah, I'd say they'd be in a bit of a jam. Needing a 17 or better to hit means they're not too effective. Although, thinking about it, why wouldn't they be splatting the zombies first? It's not like Mr. Necromancer gets to replace those at will. But let's get something in here with a bit more oomph.
Medium encounter for 7th level party (I'm assuming 4 PC's here, obviously you need to adjust for group size) - (4X750=) 3000 xp. Note, this is a medium encounter, meaning you should not be killing anyone, but, you should be smacking them around a bit. Deadly, just for my own reference is 6800.
A pair of hill giants is 5400 (Hard encounter). Attacking at range, they should get a round or two of shots before the party closes. At +8 to hit, they have a decent chance of tagging the necromancer and should be getting rid of the zombie guards pretty easily.
Manticores are one of my absolute favorite critters. At 700 xp a piece you would get 4 manticores for the same price as your allosaurus encounter. Technically a Hard encounter. But, the manticores have several advantages here. 1. They fly, meaning the zombies are out of the fight and can be ignored. 2. They get 3 ranged attacks per round. Even if it's only +5, that's 12 attacks per round. I'm going to hit. Heck, they could stand off at about 90 feet, stay out of range of the necro's spells, and bombard all day long.
2 Salamanders also fit the same budget. Granted, they'd have a bit of trouble hitting the wizard - they've got +7 attacks, meaning they'd need a 16. But, if they hit the necro, the necro has a serious problem - grappled and auto hit. And, as an added bonus, they burn up the zombie guards without having to attack them. Note, this is still a hard, not deadly encounter. Chance of dying, but, not guaranteed. Add two more salamanders, and I think the party would have a serious, serious problem.