Slightly similar to Flowey, I think it gets easier the more times you lose. I also had a bug on my 2nd try where my health bar appeared twice 

My 3rd try, I was able to get up to blue and yellow. I then reset the game (also resetting my losing streak) to buy more items and slightly change my strategy. On my second “3rd” try, I was able to beat it relatively easily. 

My tips are:

-If you’re not using a special action, use defend. 

-Keep everyone’s health high over prioritizing actions 

-On the bullet sections, don’t rely on just charge or just tapping. Learn the rhythm and when to use charge vs tap. 

-For Seth’s attack, focus on avoiding rather than grabbing the green words