This guide covers what's good and what's not for units and skills. Also tells you how to beat Zone One on insane so if you want to figure things out yourself then SPOILER ALERT.
Random Tips Edit
Stack 40+ Tech centers with the Zava Detector upgrade: Aries/Legends (Lightbringers) will become quite common. For Aries to drop a minimum score of 2000 for the Fion Drone challenge is required.
The lowest level that drops all rare units is Tesire.
100% neg resistance - for Zone One boss insane you only need to worry about negative resistance for the front of your base, and it should be 100% there. You can get this with 4 neg voiders + 3 infinite fortresses (which themselves can be upgraded to have negative resistance) or 5 neg voiders with defensive walls/un-upgraded fortresses. You only need 100% resistance, upgrading beyond that is waste of gold.
In general for max damage, number of chargors = number of attack units. (Original author was "too lazy to write the formula for this" but "they've taken high school math class so trust".)
This section pertains to units you can unlock with gold.
The Must-Haves (if your base doesn't have these then your base could be lacking somewhat):
Nyon Exist: For unknown reasons this is the only unit that prefers to focus bosses over their minions (most of the time). This is your only hope for beating Villalobos. The targeting speed upgrade increases fire rate, making it a must have! EXP gain appears to be the same for every enemy they kill so it's easiest to level them up on early levels with no other towers taking kills.
Chargors: It shoots little balls that boost firing rate of friendly units, including other Chargors. It cannot have its own range boosted. The damage increase upgrade is a must have; it should be maxed. The max health upgrade is useless. Just purchase level 1 upgrade on each Chargor. That saves you money because upgrades get more expensive each level and it should be good enough; you can always purchase more if you want.
Tech center - make sure to purchase the Zava Detector upgrade and max out gold generation ASAP. It'll be worth it in long run.
The Good-to-Haves (only because they help you beat Zone One insane):
Vira healer: one of your very limited options for healing your units. The Sanctify upgrade is useful but the others aren't so important. You'll need 4 of these for Zone One insane so might as well upgrade them early.
Neg Voider - you'll need 4 or 5 of these for Zone One insane (depending on if you have three infinite fortresses). Upgrade Team Resistance so front of your base is 100% neg resistant. Other upgrades aren't as important.
Everything else that I didn't mention. Either because they basically broken (looking at you vira savior), or because they do awful damage.
Why Aegis/Titan Barrier isn't Good: It doesn't have Healing Amplifier upgrade so you won't be able to heal it fast enough in Zone One insane against Villalobos. Use defensive walls and infinite fortresses instead. Also it doesn't protect as much of your base. If you have Vira Ultimas then this is outclassed by ZAC Assimilator which is just as tanky.
This section pertains to units you can only get as drops, but they are pretty easy to find.
Defensive Wall: Healing Amplifier is extremely useful. Other upgrades aren't important; you'll need the gold for Chargors.
Infinite Fortress: Healing Amplifier and Neg Resistance are useful. (Max HP never hurts though)
Sanctum Beacon: You'll need as many as possible for zone one insane (max 8). Make sure to max Sanctify for extra healing.
Arcane wrath: The idea is that if you only have a small space to block off you can use this which also shoots bullets for extra damage. Then why is it bad? It does awful damage, is too small, has no Healing Amplifier upgrade, and just doesn't have enough health.
Tesla: Its damage is quite low compared to Nyon Exist. It doesn't focus bosses and is very expensive to upgrade. Your gold would be better spent on something else.
Ultra Rare Units Edit
They don't focus on zone one boss so for that purpose these are all useless.
Healing wave: It doesn't need to be max level if you want to save money, as the healing is super boosted by healing amplifier on walls/fortresses. Just level it enough so that you can fully heal them when they are low on health.
Guardian Orb/Decoy: You'll need these to survive Zone One insane. Decoy buys you precious seconds to heal walls/fortresses against boss.
EMP: Buys you time to heal up against bosses or a few extra seconds when they rage. No boss should rage though if you use the base suggested later on this page.
Oblivion: Use it to buy your towers time to heal. Will also help against large groups of enemies in Zone One. Very expensive to upgrade though, so it shouldn't be a priority.
Aim: It works...sometimes.
Taunt: This attracts more enemies to the screen allowing your turrets to have better accuracy and brings your actual dps closer to your theoretical dps.
Everything else that I didn't mention. Either because they basically broken, or because they do awful damage.
Zone One Insane Edit
You don't need super rare units to accomplish this.
You must only use Nyon Exist for offense because they are the only tower that wont waste time hitting the minions.
Use Healing Wave when necessary. The Guardian Orbs get obliterated but use them whenever you can. Use Decoy, Oblivion and EMP when you need extra time to heal.
Chargor: Make sure the damage amplifier is maxed.
Nyon Exist: Make sure targeting speed is maxed. All must be Rank VII.
Healers: Make sure healing efficiency is maxed.
Walls/Fortresses: Make sure healing amplifier is maxed.
Neg Voiders: Make sure the front of your base has 100% neg resistance.
Use this base, 'nuff said. It makes Zone One on insane EASY.
14 chargors, 13 nyon exist, 4 vira healers, 8 sanctum beacons, 3 fortresses, 8 walls.(If you are using 5 neg voiders then take out a chargor)