I disagree somewhat. A large chunk of time spent grinding is in the early parts, where you have to do multiple attacks per kill. Upgrading strength is the best option when you prestige. Speed is negligible, especially in the start, and by the time you reach the point of having enough speed to matter, you can gain it quickly enough. I'd suggest ignoring speed, except perhaps the first one, to get the first few damage buffs, then upgrade to get shields so you don't die in the beginning as much.
You lose health from fighting bats, yes. However, it's a negligible amount. You lose .01 per kill, or 1 per hundred. Easy to get more, or just increase health w/ skill points.
Changing volume in windows settings isn't always an option. If, for instance, you have youtube open in another tab, it would also lower the volume of whatever video you have playing.
Aomori is so easy to win with. The skeletons give a good boost in beginning, you can easily take over the province above it, then you only have to really fight on one front. You can then take over land, while slowly building an army of giants.
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