The ledges are 10 feet above the ground. The ice is uneven ground, and you need to make a DC 15 Acrobatics check to move on it. On a success you move as if it's difficult terrain. On a critical success you move normally and on a failure you fail to move, staying stuck in place. And on a critical failure you fall prone.
In addition, when attacked on uneven ground you need to make a DC 15 Reflex save or fall prone.
Anyone beating the enemies highest Initiative of 23 may act first.
Rolls
Burning Mammoth Hunter Blue - (1d20+7)
(3) + 7 = 10
Burning Mammoth Hunter Red - (1d20+7)
(16) + 7 = 23
Burning Mammoth Hunter Green - (1d20+7)
(10) + 7 = 17
Burning Mammoth Hunter Brown - (1d20+7)
(1) + 7 = 8