I was curious about the so-called busting levels, since a higher level increases the odds of getting battle chips (I’m trying to collect Navi chips, get some new ones to complete sequences, kind of like the Zeta Cannon, and unlock the Hub Style in MMBN2). I know multiple deletions, finishing opponents off quickly and not getting hit help the level go up, but what about battles against Navis? Sometimes I take a while to beat one, without getting hit, and I get an S level, but other times I do exactly the same thing, only taking a little while longer in some cases, and the level drops to 5 or 6. What gives?
Not moving a lot also gives you a bonus but it is a pretty small bonus and you probably can’t get it for taking on navi. Also what is your defination of a little bit more time? 10-15 seconds more I think can take your busting level down 1 to 2 levels.
From what i know it’s mostly about speed,and not getting hit to get rank S on navi’s you need to do it FAST,very fast,im talking about 20 seconds tops.
I may be wrong on the time,might be 30 it’s been long since i played it.I suspect you want to fight Bass right?
I want to get everything done, actually. 
Never actually got to Bass myself,just remember try to get the navi chips 1,and 2 before attempting to get the 3 (getting 1 or S busting level) cause i just got to good fighting the particular navi that i would always get a high busting level.
Not moving much, not getting hit, under 20 seconds, not using many chips will get you an S Rank. After you fail even one of these though it can go down very fast. I’d be able to help you more if it was MMBN3. :\
Don’t different enemies have different times for an S-Rank?