Had a heck of a time with this third battle until I realized I could lunge at him (normally, most bosses are immune so I never even bothered).
You only need to know the following to beat him:
- You can do a dodge (L2), then jump, then dodge again when you land. Do this to quickly cover ground when Deadpool is shooting lasers (during the slowdown).
- You can lunge at Deadpool while he's shooting lasers to destroy the ground. The lunge range is about 1/3 the stage. Once you land the lunge, you can just mash Y for at least 5 hits (it'll be around 20-25% of his health bar)
- If he teleports near you, activate your fury, and focus on countering, and then mash Y after he lands from the initial counter hit. Once again, you should be able to land about 5 hits on him before he teleports away. Then repeat above.
It will take forever to beat him if you focus on regular combat or Drill Claw (I tried those methods a number of times to no avail), plus you're very open to falling to your death because he tends to teleport away very often. I beat him on the first try once I used the tactics above.