Gormek is the tankiest 4* defense downer and he is useful to lower defense on the first stage of the fight (tank or flank). If in a wing, once it’ll charge, your defense could have already killed heroes, making his low damaging skill pointless.
Marjana, as a fast hero, is better suited to wings positions as she can “jump into the fray” early and have a better impact on your defense. She is more durable than Gormek thanks to her stats, deal way more damage and her single target special is killer.
Magni is a single target fast hitter and the secondary effect is an increase in general defense for himself and heroes to his left and right. It would stack with Kashrek’s elemental defense well.