It really depends on the level of optimization.
That said, if they were reasonably optimized (and one, the magus, probably isn't) they would wipe the floor with a CR 7 but would still be destroyed by another CR 7 if somewhat non compatible.
You should run the fight in your head taking care of looking at the numbers: not all the parties are equal, some will have a weakness that a specific monster will be able to exploit way better than other.
It really isn't easy to say in general. Even the lowly goblin or kobold (like 8 of them) can be terrifying for a group depending on how you play them. Example:
- 8 goblins in melee: not really threatening
- 8 goblins using ranged weapons in a large distance which won't easily be covered: very dangerous
- kobolds using their own terrain (traps etc.): terrifying..
Your group isn't strong on the offensive arcane magic. Many opponents at once will be difficult for them.
One of my groups used to wipe the floor with CR=APL+3. I had to play intelligent humanoid fighters supported by bards/casters to really make it hard for them :-)
Eight goblins using ranged weapons with seven stuffed goblins (straw goblins) scattered among them behind a net or something to keep PCs from noticing exactly which ones are real and which are not.
Spruced up Hydra? They can get breath weapons or coated in metal. If you coat it in gold, it's got treasure right there too.