I like to camp them in a forest; they can aim or walk forwards, take shots, then shuffle back in to the forest's warm, protective embrace. My only Merc caster so far is MacBain!!!!, so I try to keep swapping Fortune between them and another unit.
Countermeasures makes them functionally immune to an enemy shooting unit (as long as they don't have mass anti-stealth), and since they have advanced deployment you can sometimes choose which unit you want them to face, so that can work well. Or so I hear.
Croe's + Death March - Vengeance 3" forward, forfeit movement to aim, then reform 5" back. And they like the MAT bonus. Deadeye helps too.
Running around trying hard to get the Backstab bonus generally seems to be a bad idea. Too hard to pull off and dependent on what your opponent does. Treat it as an opportunistic benefit. At best use the threat of it to try and force some difficult positioning questions.
But if you're not exploiting reform then I think you're probably better off with Nyss.
As I recall, Croe's gun prevents a Warbeast from using it's animus (although the warlock can still cast the animus), right? I mean, he can shut down any spellcasting model in theory, but it's usually a lot easier to ping a warbeast with him than it is to land a shot on a warcaster/warlock or important spellcasting solo. Well, it's something to be remembered anyway. And since Croe just replaces a grunt when he dies (unless the opponent used an RFP attack), you're not too worried about losing him.
Put the unit under Valachev, then cast Sentry on Croe with Strakhov and you can potentially stop TWO warbeasts from using their animi in a turn! Someday I shall live the dream boys!
Most of the unit stays toward the flank, except for the one guy who is going to get Death March turn 1, they receive it, they run up the flank. The Goal isn't to use them as bait or anything, they are pressing hard to the side to either make your opponent deal with or making them play more Cautions. I have 2 units of Riflemen, and a Trencher squad to act as a front line force that is more threatening.
On turn 2 you see if there is anything to shoot + Repo 5 further into their back field. On the Repo you can sacrifice a guy or 2 to be placed closer to the enemy, to be annoying or trigger vengeance when they die. By turn 3 with Vengeance you might be looking at some back stab targets. The key this turn is to keep most of your force safe and push deeper in. I still have 30 infantry standing in front of my enemy drawing their ire. Turn 4 I can typically look at hitting the enemy caster with Kiss and that can be a game ender in some situation. The big thing is play to keep the Croes around. The thought of Back Stabbing pow 10 crossbow bolts is more intimidating then reality at times, but it really pressures your opponent. Especially with Vengeance, You now have Mat 7 guys who can make a melee attack, and wont be slowed down much by jamming units.
Croe is the leader without being an officer. I don't think RFP could prevent you to elect the next guy to be the new leader, unless I missed something?
While destroying Croe normally means you just field promote and substitute another Grunt to be him, RFP'ing him takes him completely off and you have to then nominate a normal Grunt to be the new leader.
I'm going to have to pull them out again. I'm actually tempted to try out a min unit a couple times to see how useful they could be. As already mentioned, Snipe (Mags1, Ossrum) can keep them way out on the flanks and as a min unit they might be overlooked more often than not. I'm tempted to try them out with Ashlynn to give them that speed boost too. The sweet potential for a backstabbing charge under Damiano's feat and Death March seems too juicy to pass up too.
The latter scenario with Damiano is hard to pull off, and not a combo I liked when I tried it. However, the Croes in general are a great unit to act independently of your force, and were great in my Magnus2 and Gorten lists where they just wandered off up a flank, shooting and repo'ing further in to people's back lines. I'm looking forward to taking that same principle and applying it with Fiona, where Affliction seems great with them, even before they get into unit's back arcs