Endra here. I've never played an investigator, but trying:
Purrlock considers the captain's movements and where a hammer might land nicely.
◆ Devise a strategem RED: 1d20 ⇒ 16
Ooh, he's leaving that side open...
The catfolk glides just in front of him and slams it into his torso.
◆ Step
◆ Strike: Light Hammer vs. RED, bludgeoning damage: 16 + 3 + 3 = 221d6 ⇒ 3
It says for attack stratagem you sub in INT vs. STR for the attack, so +0 => +3 aside from the proficiency bonus. Let me know if I did that wrong.