OK so the engine and transmission are out of the car but still together on the subframe.... yes?
Then remove all the spark plugs and connect a battery to the starter motor and earth it to the engine, apply + current to the small starter terminal on the starter soleniod whilst you do the compression test.
Make sure the whole subframe assembly is stable (can't fall arse over tit) and the gearbag is in neutral, whilst staying well clear of any moving parts (drive belts, pulleys etc.)
To do this I'd make up a couple of battery cables with eye terminals at the engine end and suitable battery terminals at the other and a push button starter switch wired from the + to the small starter terminal.
Oh..... do make sure you have oil in the engine and gear box, you don't have to worry about coolant as your not starting the engine.
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"