Mini Split Condenser in Garage? With ventilation

Cutting a hole through brick is no big deal. You could use one of these as a disposable hole saw for $20:

Or one of these as the "right" tool, which will be reusable for $75:

Most hole saws require a 1/2 inch drill plus a ~$10 arbor, both of which will be very useful for other projects in the future, like installing door knob sets in new doors, installing plumbing, ductwork, plenums, and all kinds of projects in plywood, sheet metal, lumber, plastics, etc.

When making the hole, take care to slant it slightly downward (2%—1/4 inch per foot—is typical) toward the outside to prevent rain, etc. from finding a path indoors. Install a vent device with a damper to prevent hot/cold air from coming inside when the fan is not on, and seal the hole at the outside surface thoroughly with silicon caulk.

Slate can be cut by this type of blade too, but holes through the roof are often fussier to make leak free.

If your AC is seriously undersized, the compressor is probably already running 24/7 and blowing hot air into it won't make any difference. At max capacity an AC can remove a specific amount of heat from the air per hour. If you blow hotter air into the intake you will get correspondingly hotter air coming out. It's a zero-sum game.

In addition to closing off other rooms as others have suggested, here are things you might try:

• Block as much sunlight as possible from entering the rooms
• If you still have any old-fashioned incandescent bulbs, replace them. Ideally with LEDs, but compact fluorescents are cheaper up-front.
• Turn off any appliances you aren't using. If you need to run a small fan to properly circulate air that's OK, but you should be aware that the fans are generating heating and in that way they work against the AC, so it's a tradeoff. Some larger fans can use 100 watts or more of electricity, which is turned into heat. 100 watts of electric consumption generates about 350 BTUs/hour of heat.