I think axe is the best weapon because you would use skills most of the time rather than normal slashes and axes have the highest damage. However, the final boss is so tough to kill. I'm so lazy...
I got nearly 1000,000f really quickly (just need about 15 mins) with this strategy:
Max stats 1 and 3, buy all cannon-related items to maximum the power of cannon. Then start with a lowest possible shoot. Then when you hit the cannon, try to shoot as low as possible (but avoid being too low, or you will crash right away). Surely you will hit another cannon. This time also try to shoot as low as possible. Just continue like that. And with 3 or 4 shoots from the cannon continuously, you will get the maximum speed (I think) and reach 700,000f in a moment. So the point here is try to max out the effectiveness of cannons. It's much stronger than your normal punches.
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