Discussing with Zean and Throgin who previously opened the lid Tinkara suggest to use the magic rod and ropes to rise the lid.
Vatoris with his engineering skills suggest that to rise so heavy lid one would need a wheel and at least 4 ropes and proper tools to grab the 4 corners
[ +- ] Vatoris idea sketch
he don't have idea hot to address the lack of grappling hooks and wheel though
The next idea Tinkara has - she remembers it was enough time for people closer to the doors to leave the room while block were descending. And she have seen Ixion using dimension doors spell in earlier encounters. So this could work: one person rise lid closer to exit, while Ixion hold it at the other end. When trap is triggered the person closer to exit run away, but Ixion teleports out.
Throgin believes though that with his Barbaian agility he may manage to get out of the the room from the further distance...
Ixion meanwhile note that he could teleport one person with him, so it's 100% safe plan.
OOC:
So with 21 I give you my 3 ideas:
1. Find/construct proper tools to do the engineering project as thorsman99 suggested
2. Only two persons in, open lid, dimension door out
3. Only two persons in, Throgin hold further end of the lid, both run out as soon as trap triggered. With barbarian speed 40 he will get out safe at the very last moment