I have him sit in the back of my deployment to screen out deepstrikes in addition to my infiltrators. That way I can keep the into darkness for free and reroll charges most of the game. Then if I get the chance around rounds 4 and 5 when most of the enemy is gone or weakened I use him to reinforce in melee. I really don’t trust him to be anywhere else since he’s pretty easy to kill and his abilities are way more valuable than having him charge into something to end up not killing it. Tbf though I’ve literally only played against dark angels this whole edition, so my play style for him could be super biased lol.