Just some general tips here - but for week 20, you should be able to survive with 4 full squads of basic hilichurl. Just have 3 of them scatter, then have the final approach her as a sacrifice. She'll attack, building up her element burst. Once that's full, she'll bomb the field and run away. At week 20, your basic hilichurls should have enough hp to tank the hit. week 40+, you'll probably need elemental hilichurls deployed to survive.
As for beating her, you'll need Nycto units to actually deal enough damage to win and Dendro Samachurls to stall. Cryo Samachurls are a personal must as well, since pillaring units increases their range and damage. You can reliably get all those units by week 40, thanks to the Dendro and Cryo Samachurl trainers at Windrise and Dragonspine Camp (Dr. Livingstone and Esther).
I've already done a post prevously, outlining my current strategy -
check it out here
But, I do want to speculate a little on a theoretical week 20 path. Which was actually possible before the addition of Lumine's rations. Personally, I've never tried to win without the Cryo Samachurl, Since that added range makes it easier to keep Lumine out of range. But, assuming they are right, it's definetely possible to ge 2+ Dendro Samachurls from Dr. Livingstones training. Either by reloading and cheesing the training until you get two from a single one, or by getting lucky and getting 25 scholars by week 20. So, the only thing left to do is get nycto units from a corrupted heroine. Working backwords, we need to breed them 2+, probably 3+, times to get the nycto units we need. At 3 weeks per pregnancy, we'd need to finish corrupting them week 14, but more reasonably by week 11. To corrupt a herone, we need to pay 3* resistance+50. With their max resistance being thier star rating *10. So.. for Amber, that'd be 270 base, which translates to about 3-4 weeks to corrupt, assuming we have 400 abyss power, and taking into account Amber's barrier cost. So, realistically, we need to capture the heroine we want to corrupt by week 7, preferably earlier to actually corrupt them. That leaves us with 4 targets, Amber, Fischl, Barbara, and Rosaria. Amber and Fischl are the easiest targets to aquire, and can easily be done by week 3/4. The problem is that corrupting them this early is... a sacrifice. Amber's your only Pyro heroine, until you can get over to Liyue. While Fischl can be improved by letting her escape, but that requires that you don't corrupt her. And I don't think that sacrifice is worth getting access to Lumine 20 weeks earlier. As for Rosaria, She's going to be your only Cryo heroine for a while, just like amber. Additionally, she's a higher star rating than Amber. So she'll cost more to corrupt, and you'll have less Nycto element produced to actually do that corruption. Making her impractical. That just leaves Barbara. Hydro has another nearby heroine, Mona, that's fairly easy to capture. So she doesn't have the same longterm drawback as the other heroines. Her biggest issue, however, is actually getting her. Raiding Mondstadt is a challange, and even if you succeed? You'll be generating a lot of threat. So you risk triggering Rosaria, or potentially other enemies you aren't equiped to deal with.
So, I think week 20 is possilbe. But, I think if you want to do week 20 without making any longterm sacrifices, and without getting in over your head via triggering Rosaria, you'll either need to rush Patchi's training to get access to Shielded Hilichurls.... or rush Mondstadt on week 3 so your threat burns off before Rosaria spawns (wait week 2, spawn more hilichurl till you have 8, attack Windrise and Mondstadt week 3). I think this is possible, but it's going to be far less consistant than building up for the week 40 capture.