So if Beorn attacks Grasping Tentacle and the discarded card from its effect is a tentacle or has shadow effect, What hapens to it? Because Beorn cannot have attachments, so it would seem a good way to avoid that effect or not?

Yeah, he should be immune to the effect. Good strategy for dealing with this card.

Much better to have a useless (for combat) ally like Warden of Healing join the attack and the take the GT so it isn't around as an Enemy anymore.

Second to Dam's idea - let the GT waste it's time attached to an ally with 0 attack strength.