I cant get any of the mana gathering challenge amulets, none of the typical easy win strategies work cause I just have to keep using up all my mana to keep up with the tougher monsters.
what's your wizard level? These strategies work only from a certain wizlevel, where you can get a G7 from the very start (say 100) or when you're able to lay your hands on a lime/orange in the first 20 waves. Or you probably don't manapool enough. You need to have your mana pool size about twice the challenge amount, or a G10-G11 lime/yellow tower in 8 amps and a LOT of summoning.
As soon as your main gem reaches grade 8, unlock orange if not yet unlocked, create a G7 L/O and put in into the first or second path block, out of reach from your tower. Put amps around, fill em with G5 L/O, then unlock cyan and make a G7 L/O/C using G3 cyan, G3+G4+G5 orange, G6 lime, and combine it with your G7 L/O in a trap - it's possible that you'll have gaps in monsters' flow, and you need your mana gem to work constantly. Then upgrade main gem first, mana gem second, they should be about equal grade, and amps to be 3-4 grades lower. If your gem oneshots any monster, summon more monsters.
Umm, i think if he have grade 10-11, he can just smash it at the end, get challenge amulet and create new gem for this mana to kill last monster. It's possible without manafarm, just with a lot of summoning and high Replenish(at least 100%)
if someone can't get a mana gathering challenge amulet, try when you are a bit close to the amount you need to drop all gems on the gem anvil to reach it. i also did that.
you also can use L/O gems in traps with amplifiers. you may get above 10 mana per hit + 100% change to hit target that means actually 20 mana per hit + a fast firing speed is a real mana flow. maybe you also can add a cyan gem to it.