I decided to write a quick guide of how to defeat the first boss in 2 easiest ways I've found.
How to Defeat the First Boss
So the first boss is a giant spider with a lot of HP and attacks you cannot parry. There are 2 ways to easily defeat it.
Way #1 (takes ~5 sec.):
Before entering the fight you have to have 4-5 bombs (3 is enough if you use blade oil). You enter the room, you spam bombs, spider dies immediately, ez pz. But what if you are unlucky with RNG and cannot farm so many bombs in first few levels? That takes you to.
Way #2 (slower, requires a bit of movement):
Use as many bombs as you have, then go hit the spider a few times. Don't try to attack it in the head, just move to side and hit. Avoid spiderwebs. When the HP goes down the boss spits few spider eggs - that is your key to victory. You go and destroy as many eggs as you can, they release puddle of acid that lingers for quite some time (kill all small spiders that hatch if you are too slow to destroy all eggs till timer runs out), now your only job is to stand near acid puddle and let spider boss to walk on them. After that he dies very quickly.
That is it, but don't worry if you find it easy - next dungeon will be a lot more difficult.