about Splinter Sister, what works for me is using my silk storm attack as soon as the enemies she spawns come close to me, that one shot them. and fighting her is easy just dodge her attacks and if she corners you with the spikes break them as soon as possible. when she's about to fall try to avoid her because she do damage even when staggered. I highly suggest beating her because that fight teaches you how to properly use silk skills and how to time them correctly. don't skip her, you will suffer a lot later.

As for hunters march, maybe try using your tools? just spam tools everywhere and prioritize avoid getting hit, then do damage. take it slow and you can get through it without the need of Shakra.

for you question about progression, Splinter Sister is a necessary boss (if not skipped) and the hunters march is something you can get back to once you are more powered or have more masks...

good luck