Unfortunately I Avatar solo'd to get to chapter 8 so my normal strategy essentially didn't work, but you only need a solid flying unit (Wyvern Panne, Sumia with pocket Fred, possibly Cordelia if you do the paralogue now and level her) and someone who can safely go down the plains area (Sully, Frederick/Chrom Pocket Fred, Cordelia airlifting Avatar).
Nowi goes right, chips the DM. She has a 25% chance of killing him outright and the DM has 50% accuracy so there's a high chance she won't even take damage, though dodging isn't necessary. She gets 12 EXP or from this. DM kill puts her at 58. Nowi continues to go left and sits on the village after downing a vuln. Killing the Axe dood and the DM after 2 rounds of combat nets her 58 - 12 = 46 (2 is 92) EXP per kill and 24 CEXP, putting her at 174 EXP gained this far. Admittedly my original number of 3 may have been a bit generous (since I drafted her in that run), but she can get at least one more kill/two chips before the chapter's done for 2.25 levels or so.
Chapter 9 is mostly "kill the enemies near spawn and then fly best unit to solo 90% of the rest before reinforcements show up", so she'll get some EXP at the start and then do nothing. I figure she can cap a level here at least.
Next chapter is pretty fast, but if we're not using Rescue it's an EXP goldmine. At this point Nowi should be able to ORKO things with speed tonic and Gregor support, so she can grab the bullion or wyrmslayer for Irony.
I remember people arguing over how fast the tier player would do 11, but there is a speed wing if we do decide to take 5 turns and Nowi's definitely a candidate for it.
For the ridiculousness Nowi is capable of by Ch12, see Interceptor's playlog.