I started at Wave 16, Mix Mode.  You will have enough mana to upgrade middle buff beam to 3 stars before you start combat.
TWO BOOST SUMMONERS REQUIRED:  TWO huntress/monk guardian combo
Remember the magic numbers for guardians: 40/40;  40+ boost, 40+ upgrades.  Everything else is just icing on the cake.

Hint 1: (2) 5x stack Lightning Towers have a boost summoner using huntress & monk guardians.  It should be equipped with 70% x 4 resist armor with 1500 hero HP minimum.

Hint 2: 3x stack mages INSIDE reflect beam covering DST/LT stacks.

Hint 3: Archers/spider spread wide enough to cover stairs.

Hint 4: Archers spread wide enough to cover stairs.

Hint 5: Middle spider is approximate halfway down the stairs to keep ogres back out of swing range  End spiders are at the top of the stairs.

BRING BOOST SUMMONER WITH HUNTRESS/MONK GUARDIAN COMBO AND PLACE ON 5X LT STACK PRIOR TO BEGINNING COMBAT!
I recommend using the "END" keyset to bring this booster in, as they will not be moving for the duration of the game.

***ADVANCED UPGRADING TECHNIQUE***
Since all defenses will be upgraded, it does not matter the order in which defenses are upgraded.
It is STRONGLY recommended to use a Chicken Baller to upgrade.  It requires much less accuracy and allows the shots to keep bouncing while you upgrade.
My upgrade technique consists of pressing 5, spacebar, 5, spacebar, repeatedly and watching mana as I upgrade. Pay attention to mana consumed for the last upgrade, then you know how much mana the next upgrade will cost.

ORDER OF UPGRADING:

Start upgrading at the Stem of Green Arrow (near Hint 1) shooting toward green arrowhead.
Use your emulator to help you see if shots are going into the valley past the walkway near east crystal.
Spend one wave upgrading towers.  Hopefully all the LT's have gotten upgraded.

Wave 17 should be spent upgrading outer auras/buffs, then returning to middle to hopefully finish upgrading the remaining apprentice towers.
Stem of Black Arrows indicate where to stand and direction to shoot when upgrading outer auras.  Black Arrowhead is where shots should land.

If you did not finishing upgrading all towers/auras in the middle, finish upgrading them during Wave 18 and follow remaining instructions.
If app towers are upgraded, move onto upgrading minions near Hint 2.  Stand on Stem of Blue Arrow, making shots land on blue arrowhead in valley.

Wave 19, Home Stretch.  Hopefully you've already moved onto upgrading minions in other areas (3, 4, 5)
Hint 3: Stand on Green Stem shooting into the same valley as Combat Wave 16.
Hint 4: Stand on Yellow Stem shooting into valley with yellow arrowhead.
Hint 5: Stand on Yellow Stem or Green Stem and shoot toward corresponding arrowhead.

Now, if you have made it this far, I applaud you.  Your diligence has paid off with this last, most crucial, piece of the armor farming puzzle.

IN MIX MODE, you want to keep enemy count high by bringing 2x summoners in DURING COMBAT, then removing them PRIOR TO COMBAT starting next wave.
If you keep 4 players active, it makes combat take MUCH longer and is much riskier with lower stats due to insanely high ogre health.
If you DO NOT use this method, you are literally wasting 1500 potential enemies.  More enemies = more armor drop chances.