I wouldnt bother renting a cordless impact. That probably wont budge it. I dont know how crafty you are, but you could make a tool to fit in the balancer to hold it so you can break the bolt loose. Like this
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Funny story, one of the instructors at the chrysler training center i go to has some great stories from when he was a tech. Early in his career he worked for gm, and one of his stories involves trying to break a crank bolt loose in this very way. Long story short he hit the key at the same time his manager was walking through the shop. When he hit the key, it threw the breaker bar half way across the shop, hitting the manager in the head and knocking him out cold. Lol.