I've got weeks 3,4,9-12,14,15,16 and 18-21 to do on legendary. Looks like weeks 20 and 19 are the most difficult challenges. 3,12 and 16 seem to be really tough.
16 is one of the very easy ones. It's just a batarian, slap a full fitness cyclonic on that and engage full on easy mode. Ghost is a good map with fairly well balanced objectives. About as hard as week 21. You'll have more issues with 4, so much running from banshees, extremely annoying. And they're all gonna snipe you with warp balls from god knows where and london walls don't exist.
Can't comment on 3, I did it very early on with a not very optimal build back when I struggled with all plat solos.
18 was simple and fun, 10 was meh but I hate talon, map is decent enough to not have to auto restart if you get devices and reaper targets are the easiest.
9/11/14/15 are very easy, nothing to say about those. Full fitness/DR ACA is about as tanky as ur batarian. Reapers get much harder if they can oneshot your shield with warp balls, esp on open map like this, thankfully ACA got that covered. (which is the reason week 19 gave me a headache, I used melee damage on my talon merc and regretted every single point of it)
You're allowed to use cain mines for legendary week 19 so it's not all that difficult (something i didn't know when i did mine). Or at least that's what one of the pres's posts made it seem like, maybe he changed his mind now. Have fun with that one.
As I've said, week 20 will be as easy as you're lucky. One bad spawn will ruin your day very quickly but it's one of those get past wave 4 with most/all of ur consumables and you'll most likely succeed unless you get screwed by wave 10 kind of challenges.