The usual practice for text-based MSTs is to include all the text. That's not to say you couldn't take Ki no Shirayuki's advice and summarize any long stretches you can't get jokes out of, but I'd encourage you to try to find the humor in the blandness. I mean, you're talking about action scenes. If the action is boring instead of exciting, something has gone wrong with the writing. What stylistic or other errors have been made? What effect does this have on how you/your characters perceive the scene? Can you compare that to something funny, e.g. "watching this is like watching sloths fight"? If the text waffles on a lot in between things happening, you could intercut with snarky remarks from the MSTers to emphasize that, too, e.g.:

MSTer 1: Ooh, look, something is about to happen!"

text text text

MSTer 1: Wait for it...

text text text

MSTer 1: Wait for it...

text text text

MSTer 1: Wait for it...
MSTer 2: I'm growing old waiting for it. I think I just found a gray hair.

text

MSTer 1: There it is! A thing! It happened!
MSTer 2: How anticlimactic.

You'd want to put your own spin on it, of course. But, TL;DR, the point of an MST is for you to bring out the humor in the badfic. You can't always count on it to do the work for you. Sometimes you gotta make your own funny. {= )

~Neshomeh does not guarantee the above examples are actually funny. YMMV.