Can I repair a bad cylinder without taking apart the motor in a 2000 Monte Carlo?

Are you talking about compression readings? If so, and you only have 30 psi, engine is in serious state of disrepair. The cylinder head will have to be taken off to fix the problem-sounds like a stuck valve, a non sealing valve, or a blown head gasket. There's no way to get to it without removing the head.
Apparently you had a compression test done? If you had done or would do a leak down test of that cylinder, it would pinpoint the source of the problem-whether an intake or exhaust valve, or a head gasket, or piston or ring damage. With a leak down test, wherever the air is escaping from the cylinder points to the source: if for instance, air leaks out the exhaust pipe, you know it is the exhaust valve not sealing in that cylinder. If air leaks out the intake-it's the intake valve.
Air hissing from the oil dipstick tube would mean compression is being lost past the piston rings-going into the crankcase.