Ah, that one. I have tried V15 exactly once so far. On my one try I made it all the way through every wave without letting any monsters touch the orb, but unfortunately the ~50% mana burn guys at the end ruined my day, so I died. I'm pretty sure you're supposed to use the bolts to deal with all the heavily-armored guys, and freeze any other mob that gets out of hand. Mana is very tight on that level, although they do give you orange, so maybe it'd be possible to set up a long line of low-grade oranges to buffer that?
the low dmg ones are worthless at the end. they start healing up to 100% health within seconds.
I tried setting up a buffer of the lvl 1 hit gems with mana + crit gems behind. Once the mana+crit started to be overwhelmed, I combined them with the hit gems, which by now were at damage lvl 3 or 4. made it to the last wave but they regen too much health.
Yeah, in order to kill the ones towards the end you need probably a G7 yellow or two at least. Jury's out on whether or not mixing black in is a good idea.
Got it on my second try. There's two tricks to surviving. One is that you need to make sure you bolt the guys with 300-500 armor, because you don't have armor tearing here. For that, be sure to set your attack gem's targeting priority with the right mouse button so you don't 'waste' any shots, i.e. on swarmlings. You can also try for some frozen corpse explosion damage this way. Also, as long as the mana burn monsters don't get through, you can afford to lose even a thousand mana or so for banishing if necessary (I had some 1500 left at the end). Attack with Y or B/Y gems (I had red mixed in but I'm not sure if that helps more than it hurts).
One other trick is to put a few G1 oranges on the first 5-6 towers. They will slowly earn you some mana back to pay for their cost, help with crowd-controlling the swarmlings and tear off the preliminary shields on some of the giants.
thank you so much good sir. worked for me! yours solves some problems I was hitting, like the beacons giving the mobs buffs. having the gem right in front eliminates the beacons, and takes full advantage of the hit gem. well done!