What is the best way for an Imperium army to assassinate characters?

I run a custodes army, and I'm starting to look at brining in a small detatchement of another faction, mostly to add some cheap mass shooting to threaten hordes, but I'd also like to find a way to have a unit that is able to ignore look out sir and semi-reliably threaten characters on the cheap. The faction I'm looking at souping is Guard but its by no means locked in. Space Marines are pretty big in my meta, so the unit would need to be able to deal with their characters.

I am aware Allarus can do so, but I find them too expensive for what they do currently. I have no experience with the Vindicare, and he might fill the role, but I doubt he has the fire power to kill a character in a turn, or if he is worth it, but would love for someone who has used him to weigh in. Thanks for any help.