Leave the dust cap on (always make sure it's snug first though with a 10mm, and it's REVERSE thread, pay close attention to the writing on the dust cap) Then just loosen the 8mm. The 8mm will bottom out on the "dust cap" and pull the arm off as you turn the 8mm. So put the bolts and dust cap back in and start over. Hope this helps.
Problem: "Bike has trouble going up steep hills."
Fix: Recommend pressing harder on the pedals.
It's going to take a long handled allen, because the x-types fit so snug. I use a piece of pvc pipe on the end of my allen set to give me some leverage.
The 8mm bolt is going to push against the dust cap to pull off the crank. You have to be sure you have that dust cap threaded far enough down that it will stay put as you crank on the 8mm bolt. If it isn't threaded far enough, you could strip it. There shouldn't be that much wiggle room between the 8mm and the dust cap when you begin to extract.