Any secondary character who doesn't have an invul save would greatly benefit from the Conversion Field, which is available as soon as the character has access to Crusade Relics (i.e. after they level up once, or win a Relic mission). MoE, Sorcerer, Exalted Champion.
You can get some pretty nuts combinations. I have a Chaos Lord with Zaall the Wrathful (daemon weapon with -5AP) and am looking forward to the day he gets access to a Null-field Disruptor (ignore invul saves), though that won't be for a while.
There is a Monster Battle Honour that gives the model +2M, and +1" to Advance and Charge rolls. Put this on a World Eaters Daemon Prince with the Violent Urgency warlord trait and Talisman of Burning Blood: he'll have a 17-22" Advance move, and can then charge 4-14" with rerolls. All of this would be available when he gets his first level up.
This all being said, I am actively avoiding powergaming in my Crusade and am trying to only pick options that make sense for that character's story arc. For example, my Exalted Champion has had a daemonic patron protecting him from the first day I came up with the character, so when he levelled up it made sense to give him a Conversion Field. When he levelled up a second time, I gave him the Archaeotech Nano-meds - again representing his daemonic patron keeping him alive.
On the other hand, my Chaos Lord has proven to be completely useless, so when he finally levelled up (by default, having fought 6 battles) I rolled randomly for a Battle Honour.
Edited by Cheex, 01 September 2020 - 10:25 PM.