The Guardian Scouts are prepared for many things. Truthfully, a kid punching an arm to wrestle its weapon away was not one of those things. The guardian emits whirring that somehow conveys it's surprised, and loses its grip on the spear! Zuko makes good use of the spear by trying to stab at its eye, the guardian moving aside, the spear leaving gouges into the outer casing. These guardians are very vulnerable to their own weaponry!

But the guardian grabs the robot arm hanging from Zuko's leg and pulls to make him fall to the ground [14], and begins pulling from him, dragging him along, towards the spikes on a side, placed there by the one controlling the hallway, inexorably dragging him along!

Sebastian's bombing the spikes plan is a good idea! The bomb detonates, its bluish explosion searing spikes. They don't break but they certainly look fragile now! The guardian handling the hallway, unhappy its weapon is getting damaged, sweeps the sections underneath Sebastian, making him slip, and places the fragile spikes above Sebastian, the movement breaking some, falling towards him! It's been pretty telegraphed and only one section moved like this, though, [6], giving him plenty enough time to make a move.