In both game modes you can't really parry guardians, they continue attacking as if nothing happened, you also don't get the melee damage increase afterwards. That way it might teach new players that parrying guardians only blocks 1 melee, also you can't practice remembering the parry stun duration unless you try it in a bot game, there it works as intended.
Also the ability to parry troopers has been removed some patches ago, but you can still "parry" them in these 2 gamemodes. But as with the guardians the parry does nothing, no damage increase and no stun.
Also the ability to parry troopers has been removed some patches ago, but you can still "parry" them in these 2 gamemodes. But as with the guardians the parry does nothing, no damage increase and no stun.