He's trying not to use her as an anchor as much as he can, bracing himself on the rope rather than tugging himself up - there's no point dragging her down with the snow stuck to him.

But soon Cassandra will feel the rope go slack, with a satisfied, breathless huff from Arthur. "Right." First bite successfully swallowed. "Alright - I'm going to jump, so it's going to be a hard tug." He steps back a little, as best he can, for any space for a run-up. "So- so on three, brace yourself - one, two--"

And "Three-!" comes out strained as he does, indeed, jump on cue, a running start to try and kick up the uneven wall, yanking on the rope to boost himself just enough to get his fingers over the edge, and then his legs are scrabbling against the wall to push himself up over the finish line.