Ooh, last section definitely requires a bit of build crafting, especially if you're doing the purple route (unless you max it out). Some hints would be:
- make sure you're completing tasks (bottom right blinky, those give bonus xp)
- the final skill for each stage in the ascension tree is very powerful, and you'll probably need to have one equipped (depending on which route you want to go) and synergize with it to beat the final boss
- final boss activates their skill at 75%; bursting is definitely an option!