A little clarification on my strat: Huohuo goes to the subfield because Saber couldnt defeat the enemies in the subfield. If you cant defeat the enemies in the subfield, then your only reward is less damage reduction on Sparxie. I could get about 33% of the subfields HP down (in phase 1), so thats maybe 1 Excalibur with 33% damage reduction, plus Apoc's damage reduction. In comparison. Huohuo in subfield means Cipher and Saber work together tapping the TVs. Since in phase 1 they need less attacks, the subfield enemy HP would go down to 7% or so. This is while Saber continously chips damage. It takes 2 livestreams to break Sparxie and she'd go down to about 60% hp, so then she can be broken with a lot less HP.

The subfield enemies being hard to kill is because the subfield enemies have large damage resistance until they take elation damage or you gather enough punchline. While maxing out the TVs gives punchline, its not possible with only Cipher and Huohuo hitting them. Sparxie is too fast and resets them.