To beat Yurmala isn't as daunting as you'd first think.
What you need to do first, is use your grapple hook to get up to the circular walkway. Wait patiently for Yurmala to climb up where he pauses long enough for you to make a move.
Equip the lightning power for your gun. Look at the platform that protrudes to almost the middle of the arena. Use your lightning gun to connect the node to the one on Yurmala's shell. Then it will fall backwards and lie down on its side. Jump down, equip the normal bullets and shoot away at the weakspots. Rinse and repeat until it dies.
This is not a hard battle, but l9ng winded as your AI team mates will just follow you around and do absolutely nothing