ON Electrical Code: 240V line from panel to exterior (fused / nonfused subpanel) for AC
12 gauge wire is sufficient up to 20 amps.
240v (except for stove and dryer) needs 2 conductors and a ground (no neutral), so 12/2 should do it. The black and white will be the two legs, then the ground.
You must have a disconnect at the unit. Run your line from the breaker box to the disconnect. If the line is inside an attic or wall or under the house it is considered “protected”. I ran mine under the house,stapled to the underside of the floor joists. At the disconnectI drilled a hole through the outer floor joists the outside. I drilled the hole large enough to accommodate 1/ inch rigid conduit through the hole to the outside, and rigid conduit up to the disconnect. I haven’t gotten past this point, but some sort of whip will run from the disconnect to the unit.