I've read in a few comments on the Pixel Dungeon facebook page that the game is 'easy' if you farm health potions. I know that Flies and Bats drop health potions, but when I stick around on those levels to farm them, I lose more health from starvation and fighting other monsters than it is worth to get the potions. Is there a trick to this?
You'd need to be strong enough to survive a long encounter with flies. Using a low damage weapon seems to be ideal with the flies, as well as one that is not enchanted. A melee hit will cause a fly that has not reproduced, to reproduce. Any ill effects seem to translate to the new fly, which is why enchantments are not the best idea, as you want to have the chance to strike the new fly before it can die of ongoing-damage (so that it will split off and form a new enemy). The health also seems to translate over to the spawn, thus the lower damage weapon will yield more flies. And each fly has a chance of dropping a health potion.
Another method I've seen that can be beneficial is having a wand of regrowth and harvesting the dew drops (to hold off on needing to eat as often, for example). An added side effect is finding enough seeds to hopefully brew a few potions. This is particularly useful for the Huntress as she gets a bonus to her HP returned from a dew drop.
If you can manage to make it all the way to the first shopkeep before discovering which potion is a health potion, you can buy them cheaper. There is almost always one or more for sale. It is obvious at that point which potion is health as it will at that point be the most plentiful one you've encountered (your inventory + shop inventory). Though, not always possible to beat Goo without a little health boost.
Unless/until you have some seriously high power gear, farming bats is not a very realistic task.
Can't comment because of a lack of reputation.
However, using GhostToast's answer.
Flys can continually reproduce, it does not matter if they've reproduced once Don't equip a weak weapon. Just un-equip your weapon.
Barehanded attacks are only stronger than cursed weapons, but cursed weapons are harder to hit.
Also, when encountering the first fly, stand in a door and force that fly to reproduce as much as possible. You should only be assaulted by three flies at most. When there are no other adjacent spots for the fly to reproduce, walk diagonally to the corner of the door.
A fly will follow you into the door, hitting weak flies that are in the door frame should make it so that you mostly will only be fighting one or two flies at a time.