Well from my experience, the Silver Rapier upgrade is the hardest upgrade in the game as far as I know. The Gremlin towers have 12k-13k health I believe. And there are a ton of undead units to combat. Stacks of 500 ghosts and 200 undead knights amongst others.
I was able to upgrade a level 4 artifact to level 5 a lot easier than this particular level 3 rapier, and I only managed to do so at a much higher level. So this exploit may only appeal to people who have reached late game. And by the time you reach late game the desire for runes and magic crystals isn't all that high. But your mileage may vary.
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