Base sword deals 4 damage, and base skeleton has 16hp. Each sword upgrade adds 2 damage.
4 damage 4 shots
6 damage 3 shots
8 damage 2 shots
10 damage isn't enough to one shot, you need 3 extra ranks before one shotting. You might want to consider other upgrades rather than trying to one shot with sword... on the other hand, the skeletons get more hp as they gain ranks too!
I get the idea, but it still feels weird to buy 2 upgrades and have absolutely nothing change. Since you can't see the skeletons' hp or how much damage you are doing, it just feels like you wasted two upgrades. I don't know if there are any other cases in the game like this one.
In any case, I love this game so much, especially the art!