I would choose Shadow Sneak
It may not be as powerful as Shadow Punch, but it has priority. So if you and your opponent have 1Hp left, you can you Shadow Sneak to take them out.
PS: Instead of Dig on Muk, I would replace it with Power-Up Punch so you can boost your Attack and sweep with Shadow Sneak, or replace it with Curse, since it's Speed doesn't matter because Muk isn't the fastest Pokemon out there, plus you get an Attack boost for hitting hard and a Defense boost for bulking up. Also make Muk Adamant. You'll need the power
Hope I helped!