You (or a helper) will have no problem holding the crank steady if you use the impact driver. The torque is delivered in very short impulses. I easily hold a wrench to a fastener while I “impact” the other end. The helper will also enable you to concentrate on not having the socket slip off the bolt and mess it up. Strangely, in another recent thread, I stated that there were few, if any, bicycle applications that required this much force. I clearly didn’t consider this instance, which has “impact driver” written all over it. That being said, however, give the penetrating oil a bit more than 15 mins to do its thing before you go medieval on the bolt - the less hammering necessary, the better