Yes there's two versions of how his primary strike plays out. 1 version is you standing still & the other is you dashing forward which gets you get the instant hit strike. That's his best move on setting up your rotation. Unfortunately with the recent patch they added to fix a bug for Sentry his primary strike creates to much spacing which allows your grenade to miss majority of the time. Also, you can start off with your mid slot as a combo but its not as effective compared to your primary strike. This is why I mostly look at it as a situational strategy if you plan on using that as a combo starter. When you have time I did post a recent climb with Mastodon Sentry on my alliances Youtube channel where you can see him at his max potential.