How do I perform a stealth, non-lethal takedown on an exosuit enemy?
To perform a non-lethal takedown of an enemy in an exosuit, you must stun the enemy first with an EMP weapon (e.g. gun with EMP ammo, stun gun, EMP grenade). After stunning the enemy, you can perform the takedown the way you do with other enemies -- by sneaking up behind them an pressing the takedown button.
If the weapon you use to stun the enemy is silenced, or if you're far enough from other enemies that they will not hear the weapon, your stealth should be maintained.