Played the web version, followed the enemy to the tunnel and got hit, but using my shield I tried to catch up to him because I couldn't hit him with my spears. So I'm running with my shield and another guy joins him and then another but I'm still running shield in hand unable to be hit, then suddenly they clip through the terrain and their spears are blocked the moment they throw them and I just jump out of the cave. Did the same thing to the same effect on the next stage and then for the boss I thought it was a cave entrance so I ran into it, realized I couldn't go down so I started throwing spears, realized that I wasn't being hit by the attack because I was behind where it spawns from and killed it easily...
Basically I managed to complete/break the game by running past everything with my shield, finished it in under 3 minutes on my first play-through. The only reason I found this bug is because the enemy wouldn't give up ground when I was chasing him to get closer to spear him to death, after that I realized shield = OP.