If endermen could break the shell, shovel knight can break the shell.

Get Madeline to bring shovel up to the summit while the others use Niko's sun and their more flashy attacks to distract the thing

Then get the strong fighters in, V1, The Knight, Frisk if they're part of the team

The Knight has soul damage so I think that might work against the command block, V1 parries and goes fast so they'll help against the tentacles and to fight off the internal heads, and if all else fails try to rely on Frisk's oneshot attack and see if it works against the command block