Offensive: These are about as deadly as you can get when countering tanks with their range and damage. They are also good against any other infantry unit being able to kill them with one shot when fully promoted. Unlike the Hastatus Sniper, this deals much more damage, allowing it to be a threat to not only tanks, but also serve as a recon killer, too. When they group up behind enemy defenses, they will be unstoppable. Make sure you give them an armored escort for recon units, a strong tank to protect them from other sniper infantry, and an AA escort for helicopters. Use an artillery unit alongside it, too, as you may need extra firepower to take care of the long-range units can easily kill your Prime unit. Use armored units in front and these in the back so that they can group up behind them and wreck havoc.
Defensive: Rail gun units are a lot less effective against recon so take your pick of any recon with an anti infantry cannon though beware the Prime still has long range so something with good range or speed is recommended. Helicopters can't be targeted by the Prime and some really long range artillery can kill them in one shot. A recon artillery like the Sejanus works great. A Theseus used to be able to kill them, but this unit got a HUGE buffer in the update and can now destroy them in one shot before it can land even a second shot off. If you don't have a Psiloi type infantry-killer yet, it's strongly advised to NOT go against an army with these in it.