If Aran'gar is as strong as Moghedien (in absolute terms), and Graendal is about as strong as Sammael/Demandred, I wouldn't be surprised if she could do all she did.

Delana is pathetically weak. She's not much stronger than Liandrin, and we saw how easily Moggy shielded her even when weakened by Healing. Plus, is it very likely Delana wouldn't dare hold the source in front of the Foresaken unless given permission. Thus, Graendal would have had to use very little to shield her.

If Aran'gar was definitely weaker than Graendal, Graendal would still be using the bulk of her power to shield Aran'gar. I'm arguing she's strong enough that she can make a small Gateway and shield a non-channeling Delana at the same time. It isn't so very impossible.

If she had her very weak angreal, this becomes still easier. In her next scene, we'll know if she has an angreal, clearing this confusion up.

But either way, if Graendal was say strength 95, and Aran'gar was about 70, then this scene works.

From Nyn and Moggy's battle in tSR, we know that you don't have to put your full strength into shielding someone as strong as you (since both women threw a shield at each other, and had enough left to block the other as well). Which means Graendal probably used around 60 strength on Aran'gar, around 15 or so for Delana (since you need even less to shield a woman not channeling) and used the other 20 strength for her Gateway.

In all probability, she used even less for the shield on Aran'gar.