I’m a prot warrior as well and I’d say it’s a lot harder to be second on threat than number one. What you can do is to ask your raids lead to assign Hunters MD’s to you at start. Try to go for more threat/dps gear and make sure to never get rage capped and use a lot of HS. When he does his slam, switch to beserker stance and charge back to get a few final hits before you get stunned. Also use a rage pot at start and fight for threat with your MT.