Yesterday I have finished my first Endurance at lvl about 55, and after I figured something out, it was a breeze.
Set monster type to Giants only – each bomb thrown on a wave will add a LOT of mana for you to get by killing mobs. Get used to bombing each wave as much as you can before monsters begin to walk past your tower. I usually bomb them once with gems abut 3 lvls lower than my main gem(maybe for the first twenty or so waves I used only Grade 1s).
Start with Wild Gem, to already have a pretty strong defence at the very beginning, but then:
Definitely use Y/L/O trio as your main weapon. Unlock them all and use them no later than at about wave 40.
Your setup should be like one tower + 5 or 6 amps. Use YLO trios in them as soon as possible. After you are settled here, start building a “mana farm” of two traps side by side with 4 amps on both sides. Put it in a place where every moster HAS TO pass. It should look something like
A – amplifier, T – trap, X – road
Remember to bomb the waves once, but with progressively better gems. Do not bomb a wave twice with grade 1 if you can bomb it once with grade 2. You will get the same amount of monsters for a bit lower mana cost, but they will be a lot stronger.
Why I put mana farm in "" ? Because It will soon be your main weapon. YLO gems around lvl 22, 23 easily counter any number of max stats monsters, while generating about 200 000 mana each hit(and they have firing speed in high 300s if amped properly).
Remember to manapool whenever you can, it really boosts your mana income in the middle, before wave 100 get a multiplier of about x20. When your traps get to lvl around 24 you can Shift+click on manapool spell to set it on auto, it won’t waste your overflowing mana and it will raise your mana multiplier.
At wave 155 I got bored and started to go for battle amulets – having four CB shrines, I gembombed several waves to get about 400 monsters in range of shrines and dropped lvls 24 in them – no single monster, each with about 2,5 times of max hit points(due to bombing) survived.
After completing all battle amulets get back to using tower gem. It won’t pump your mana that fast, but will easily ill any number of mobs before they even make it to screen. Launch all the waves remaining(there is up to wave 1337) and enjoy your score!
I ended up with 6 gems lvl 24 in two traps and 4 amps near them, which later got thrown into tower and its amps. I managed to do it all before wave 245, then, after getting all battle amulets i stopped bombing waves and just let it all finish by rushing all the remaining waves. My last mana multiplier was 623,75 ;)