This is a near-total clone of Nodebuster, as another commenter notes, but for a free web game, I don't think that disqualifies it from consideration. Effort visibly went into a full reskin -- giving it a fantasy monster-slaying theme -- and it does have some innovations, including an entirely new equipment mechanic which lets you boost stats and customize your build toward one of the game's damage types.

There's one or two QOL features which are missing (notably, buffs to resource pickup radius), but there are also a few that have been added, such as a grace period after the boss kill to allow you to pick up dropped loot. The game is also far shorter than Nodebuster, being finishable in 1-3 hours and with only 10 levels, and balanced so that the final boss requires filling out most of the upgrade tree.

If you enjoyed Nodebuster, this scratches that itch admirably, and if you haven't played it, this is a free chance to see what the fuss is about.