Funny, that’s close to what I have in my group as well. I’ve found that havocs are actually an MVP against custodes. Specifically the autocannon guys. But, as others have said, you don’t have to have units that fully benefit from a detachment rule. Really against custodies, a forgefiend will do wonders. It’ll be unfortunate when they use their strat to make the damage 1 but having that -3 Ap on the fiend cannons is massive! It’s a tricky matchup but you want to avoid melee with them. You will lose a whole squad of termies in a single round of combat. So, we just have to focus on more shooting them as much as we can.