Best way to run power cable through firewall without damaging it?

You'll want to use a firewall grommet or pass-through rated for electrical cables - they're cheap and seal the hole properly so you don't mess up the fire rating of the wall. Drill your hole slightly larger than the cable diameter, feed it through the grommet, and use fireproof sealant around the edges if needed (some grommets come with it built in). Never just poke a cable through and call it done, because that gap defeats the whole point of having a firewall. If it's a load-bearing wall or you're unsure about local codes, might be worth a quick chat with an electrician to make sure you're doing it right.