Another way to do it is to get a lime-yellow dual and simply keep one gem and sweep the monsters with it (maxed lime and yellow mastery). This way, you should also be able to get the journey amulet where you have no more than 2 gems at any time and spend 5,000 mana. You should have enough starting mana with wild gem at this point to unlock the gems.
What level wizad were you when you did this one? Which skills did you use and did you buy the premium upgrade? I've been stuck on this level for quite a while now too. I'm level 68 and don't have premium but I'm thinking of getting it
Thanks Gremlion, I've been struggling too When I sacrificed a high level "wild gem" I got enough mana for a level 4 lime-red. I managed to beat the level and I was level 67, so it's possible. For lower levels, a red-yellow dual might be able to beat the giant monsters I think. I don't think you need any particular skills except wild gem.