By wave 100 your mana pool should be around 250-300k
What you need to do is build a mana farm at the beginning of the level. If there is more than 1 entrance for monsters put it just after their paths converge.
I typically use a straight section of path and set up 3 traps with lime/orange gems that are surrounded by amplifiers on each side (6 total) with L/O gems. The trap gems should be 2-4 grades higher than the amplifiers.
This setup will get you mana at an extremely high rate as the mana gain is also applied to the multiple hits lime gives you. Because you gain mana so fast you can upgrade extremely quickly. I can usually get to level 12 or 13 for all gems by wave 70 or so. You can extend the mana farm as far as you want but this small one will get the job done, cost very little (comparatively), and most laptops will struggle to run smoothly even with just these 3.
Protip: At this point you should shift-click the extend mana pool spell. It will automatically cast when the mana bar fills and extend your pool at the fastest possible rate. If you've never done mana farming like this you will be blown away by how quickly the bar fills.
3 level 13 gems surrounded by 6 level 13 amplifiers will be able to take you to about wave 190 before anything is able to get through. So you have between wave 70 and wave 190 to build a killer gem towards the end of your path uninterrupted. You can upgrade your mana farm along the way but you'll see rapidly diminishing returns. After wave 220 or so it doesn't matter how high level the gems in the traps are, monsters will start to get through.
A level 21 gem surrounded by 8 level 20 gems will be able to kill monsters faster than they enter the field, even at max HP. Then just ctrl+click the waves and sit back. You'll get between 1-5 million XP depending on the map and the number of amulets you earn.