Is it just me, or does Red High seem to employ Rubberband difficulty?

Yes.

On the nurse stage I just avoid the brown haired nurse. On the assassin stage I stand in the middle with tornado. @_@~

I hate these timed stages. Makes you go so much slower. Imagine having to deal with that with 15 characters

Nurse stage = use Dark Sight/Infiltrate, spam CSB/SSK or just plain jump around. I simply hate this stage when it's like... the 10th or so. You simply have to pot every hit you take.
Assassin stage = stay afk for 2 min while sitting at the top of the room. There's no point in killing anything because it makes no difference in the timer and killing them wields you 1 EXP anyway

EDIT: about the time needed to clear all the 12 stages: well, I can safely say that it can be pretty easy even if you don't have a lot of gear if you have a high DPS move.

For instance, I can clear it easily (i.e. I don't need to transfer over equips) with some of my chars that use only a CZak, TT medal and a statless weapon whereas I've had pretty bad times with my Bishop, BaM, DrK (and I think my Jett as well, but I haven't really tried that 'cause it's 10x so it has some lower level equips on it all the time but it barely clears it with only a minute to spare) that used that same setup.

I mean, it's so much easier to make it through with a Sair spamming RF than with a struggling DrK with DI/Sacrifice while having the same level of equips on both of them.