It depends on your party. They don't look very optimized. As others pointed out, very melee heavy. At that level, they should have items that can counter the invis. If they didn't, well, that's just wierd. A Scout's Headband is only 3400 in the MiC. Potion of See Invis is 50 gold. Maybe 300. Whatever.
The shinobi? Sounds like the worst of all worlds. A monk blaster. What the fuck were you/him thinking? Seriously? Was it an experiment in mega-fail?
Someone in that group should be playing a real caster, or you should be handling them with kid gloves.
My 5 player group is all ECL 14 right now, limited to spells of 4th and below, and we would have raped that encounter with no lube. Admittedly, we mostly play full casters/gishes. So there's the spread. With a completely optimized 4 player party, I'm sure that encounter could be handled at level 9-10.