Snakes have a very high evasion rate, which is why they're so hard to hit. When a enemy goes through a door, (which you can see in the image if you look closely) you will always get a guaranteed hit on them on the turn they go through the door. Hitting sleeping enemies will always result in a surprise attack as well.

If you're unclear how to do this, try the following:
-Get a snake's attention and have it follow you
-Go through a door, and wait until the snake enters the door
-Hit the snake, usually killing it, but you might have to do it again if it survives

Using doors is also great on other enemies that dodge a lot of hits, like ghosts or the giant crab.
If an enemy does not see you, you can also hit them for a surprise attack. Additionally, the Rogue's cloak is a very good way to sneak attack snakes, as any attack while invisible will function as a sneak attack.

Projectiles (and magic especially) are great at dealing with these enemies from a distance. Throwing a weapon at a sleeping snake is a very effective way to deal with them easily, and the Warrior's rocks and Rogue's throwing knives are both great starting weapons that can one shot asleep snakes.

Probably the most effective method however, is magic. Magic never misses as long as you have a clear line of sight, and the Mage's initial equip staff will make short work of just about any snake (or really any other enemy in the sewers).

If you're really, and I mean really, desperate: Use weapons like liquid flame potions, bombs, or sheer stubbornness (and half your health bar) to kill snakes. Snakes only have 4 HP, so they will fall quickly to things like fire, poison gas, certain seeds, bombs, scrolls of retribution, y'know, things you can totally easily find all over the sewers.

Then again, you can always try to run away instead!

Really though, it gets easier, don't expect to have an easy time with the game, but make sure to have fun with it!

also, as far as I can tell, this is the first guide for the steam release, so woohoo! Cheers, Evan.