Sniper's guide to bosses:
Goo
A. Explore the map, find a long straight corridor/good spot, wake Goo from far away, lure him to there, position yourself and plant a blindweed, kill him easily. Blindweed blinds and cripples so all your attacks are surprise attacks so they will always hit and cripple makes movement take two turns. Add firebloom/sorrowmoss although its not needed.
B. If you don't have blindweed or don't have any excess(you at least need a total of 3 for all boss), follow the first way but replace blindweed with stormvine/levitation pot. Although this might take longer because some of your attacks may miss since Goo is not blinded anymore.
C. If you didn't got those mentioned above, you may use your honeypot, or just do the usual boss fight with earthroot/backing up when goo is charging/use doors for surprise attacks.
Tengu
A. Use blindweed again. Before entering the room, plant the blindweed in the center(this is a 5x5 room I'm sure you know where it is), step into the room to spawn Tengu, throw something to trigger the plant(stylus or dew vial), and hit him until the 2nd stage(Tengu is blinded so he can't use shuriken). On 2nd stage, finish the maze, locate Tengu and try to position yourself so you can hit him while he can't(this is hard to explain but you'll learn from experience through tricky shots).
B. Plant earthroot for defense and just him him with the boomerang. Use honeypots if you have.
DM-300
A. If you have a blandfruit and earthroot, cook them and eat them before the fight. Earthfruit gives you the ability to root enemies when attacking which means DM gets stuck in one place while you are sniping him from a safe distance. What's also good about the fruit is that the root effect stacks so don't worry about the buff expiring(30 turn buff).
B. Find a place with few trap tiles(water is the best here), either use blindweed/stormvine/levitation pot.
C. If you're actually that unlucky and never got blandfruit/earthroot/stormvine/levitation pot/blindweed, just use honeypots although you might need 2-3 of them.
Dwarf King
A. Blindweed again. After entering the room, position yourself so you can hit him without the statues blocking and throw the plant on the left pedestal before DK steps on it. Because he is blinded, he can't summon and will just stand there until he die(pretty sure you can kill him before the debuff expires).
B. If you have another blandfruit, cook it into earthfruit and eat it before entering the room. Similar to DM, he will just stand there rooted until he dies.
C. If you have a lot of bombs/liquid flame pot, now is their time to shine. You can use levitation to set up the bombs since bosses won't spawn when levitated(except Goo). Surround the left pedestal with 1 bomb each(use throw when setting up the bombs and not light and throw). When DK spawns, hit him with boomerang until he summons, then throw the liquid flame at DK to detonate the bombs causing chain reaction making it possible for DK to instantly die(I did it once).
D. Throw toxic gas/levitation pot so the skeletons gets vertigo. Then proceed to hit DK until he dies, don't worry about the skeletons since they are vertigoed and toxic gas will deal with them.
Yog
A. Before moving from the stairs, use mind vision(and magic mapping if you have extra), throw a frost potion on them to freeze them(not really needed but I do this so both fists get paralyzed on a single potion), then throw a paralytic gas, kill the fists first(after you kill the fists the paralytic gas will end in a few turns). You can throw another paralytic gas at the eye to paralyze the larvae, and throw a toxic gas. Just hit the eye and let the gasses take care of the larvae.
B. If you never got any paralytic gas, before moving from the stairs, use invisibility and mind vision, hide yourself in a corner, and spam spectral blades.
If you did it the right way with the best way(1st), you won't get damaged at all(except for Tengu's first shuriken). Happy hunting the bosses.