Life Crystals only drop in the Advanced/Expert stages of the Current Event (Not Re-isssued.)
Ampies can drop in the Expert level of the Current Event. Regular level 5 Manyu can be found in the current Intermediate/Beginner stages of our current Event map*. (Can only confirm the Intermediate, but someone in Discord mentioned Beginner as well.)
12-X *Be it 12-3/12-5* can drop Ampies and gifts but never Manyu from the base mission. It does have a high chance to spawn a SG, but personally it seems like 12-3 spawns SGs more regularly than 12-5, which seems to prefer spawning raidbosses-this could just be my own bad luck though.

While the rate of Ampie drops is lower in 12-X compared to Expert Event maps, you have to consider that 12-5 is half the stamina cost. So RNG plays a factor across multiple runs. Expert Event maps also drop LGs, so if you get a rare chest in that map it's only a 50/50 chance of an Ampie or something along those lines, but 12-X drops only Ampies from rare chests, so if you really don't need the Skillblooms from the LG exchange then farming 12-3 is probably the best way to mass accumulate Ampies normally, because you just more runs per Stamina. It's also not that uncommon to get 2 Ampies from one 12-5/12-3 run, at least I think so. Usually every 20th run or so you can find 2 Ampies per run.

I know the missions from 13-X can also drop Ampies, but they have less chests per mission so they're not really worth farming for them. Cannot confirm missions 14-1 onward as I haven't gone that far. (Trying to full clear chapter 13 first.)

Edit: BIG NEWS! They fixed the Disconut typo. Nameless finally getting his shit together.