As you mention this there is an explosion midway up the rise, and debris (not to mention a huge spiked tree trunk) come raining down.
It's a trap!
Reflex saves from those climbing, initiative rolls from everyone!
Burnt Mule init
Rolled 1d20+2 : 7 + 2, total 9
Mules Mooks
Rolled 1d20-1 : 12 - 1, total 11
Rolled 1d20-1 : 3 - 1, total 2
Rolled 1d20-1 : 18 - 1, total 17
Rolled 1d20-1 : 16 - 1, total 15
Merc Leader Init
Rolled 1d20+2 : 18 + 2, total 20
Ref save
Rolled 1d20+8 : 7 + 8, total 15
4 merc Init
Rolled 1d20+1 : 18 + 1, total 19
Rolled 1d20+1 : 16 + 1, total 17
Rolled 1d20+1 : 14 + 1, total 15
Rolled 1d20+1 : 3 + 1, total 4
Ref Saves
Rolled 1d20+3 : 18 + 3, total 21
Rolled 1d20+3 : 1 + 3, total 4
Rolled 1d20+3 : 18 + 3, total 21
Rolled 1d20+3 : 1 + 3, total 4
Cleric Init
Rolled 1d20+7 : 7 + 7, total 14
Badass Robot FIghtin' Mans Init
Rolled 1d20 : 1, total 1
Rolled 1d20 : 11, total 11
Ref Saves
Rolled 1d20+5 : 1 + 5, total 6
Rolled 1d20 : 4, total 4