Short and sweet. I was asked in a thread to expand upon my high Endless scores so I'm going to do that here. This is a guide about Endless rather than how to complete each map and the towers mentioned here are going to have different uses than they would have if you're just trying to beat each map. A good example of this is the Sniper. In a normal game, this thing is S Tier. A few random towers + a Sniper tower going down the Siphoner path can beat every single map. On Endless however, the Sniper damage falls off as it doesn't have any inherent scaling relative to how tough enemies get and the Sniper can't debuff enemies in any way. Also note that this is with all Anarchy Mods on. Without Anarchy Mods just filling the map with Wind Mushrooms and Fire Curses is enough to kill anything until the end of time.

Towers that can target the killbox:
Forest - Decomposition
Explosive - Fire: Elite Extermination - Wind: Hail of Fire - Shock: Cluster Count/Hail of Fire
Sniper - Siphoner
Gatling - Titan Slayer
Curse - Pestilence
Divine - Wind: Energy Wave - Fire/Shock: Divine Champion
Mushroom - Deathcloud
Cyclone - Calamity
Laser - Iconic Trio>Find Enlightenment>Fear No Elite - This all depends on the map. If you can get a beam or two to intersect your killbox, take Iconic Trio. If not and you can place the tower in a spot to absorb a bunch of empty tiles that will then be surrounded by other high-value tiles, take Find Enlightenment. If not take Fear No Elite.

Towers to skip:
Fire Forest and Lightning Explosive don't offer much of anything. Fire Forest confuses me. It's arguably worse than a regular Forest tower and is most definitely the worst tower in the game. I don't understand why it gets worse as you level up your Fire Hellement. The first and third tiers are great but the second tier locking you into 1 attack every 2 seconds is just terrible. Lightning Explosive is fine if you can squeeze it into a position that hits the killbox but it doesn't offer anything outside of damage and it's usually just worse than adding a regular Curse tower.

Fill out the remainder of the map with Sniper towers, Divine towers and Forest towers if you get big enough connected clumps to take them down the Leyline Manipulation route. It's also important to note that the slots you get on the side of the screen for ten activated abilities are the only ten you can have. If you place 11 Sniper tower with Siphoner, only ten will activate. You're taking 3 Siphoner, 1 Lightning Curse and 1 Wind Cyclone active/s already so you only have space for 5 more Siphoners if you go that route. Full Auto is fine for distant Snipers behind the killbox but for anything infront of it the lack of piercing will not be good.

Build the Divine towers with +range unless you can somehow give all three cyclones both the ability to target air permanently and the ability to target the killbox. Build the Lasers with increased range. Wind Curse allows all towers to hit Cursed fliers. Build Cyclones with armor reduction. Build Gatlings with flat damage bonuses so Titan Slayer has more damage to multiply. Tier 2 Mushroom upgrade depends on if you need the placement range or not.

Green Lines = Enemy Path
Red Lines = Blessings
Grey Circles = Potential placement of Fire/Lightning Divine Beams
Yellow Circle = Placement of Wind Divine Beam
Blue Hexagon = Killbox

This is a very basic example of a killbox where every tower placed can hit the killbox. Place sniper towers anywhere within the grey and place a basic Divine tower anywhere with a beam built for damage centred on the shock spore to allow the Fire Curse some increased range. The enemy path can be adjusted.

The goal here is to combine the air and ground routes to as close a point as possible to not waste damage. You want many things to combine on one point: All three Curse towers, all three Cyclone Towers, all three Mushroom towers(Referred to as Spore in the image), within the Divine tower Energy Wave range, within the Wind Spore stun range, within the Wind Explosive tower initial stun range and within the Wind Cyclone and Wind Laser stun range. You also want a fire tile placed on the box, a lightning tile placed right before it and wind tiles after the box and before the lightning. Then you have to worry about towers that exist to deal damage rather then these that are here to debuff. This is a lot to get right and is possible on 6 maps as far as I can tell: Meadows, Tundra, Forest, Toxic Boneyard, Dojo and Ruins. I've had the most success on Tundra but they all work.

Here's what happens to enemies who hit the killbox:
Initial 2-second stun from divine beam > Wind Mushroom debuff starts building up > enemy is hit either by Mushroom stun or Energy Wave stun for 2 seconds > enemy hit by whichever stun they weren't hit by the first time for another 2 seconds > enemy moves slightly into the divine beam and is hit by the 5-second stun of the Wind Explosive > enemy hit by a barrage of 2-second stuns from Energy Wave, Wind Mushroom, Wind Explosive, Wind Cyclone and Wind Laser(Get a sense of which Hellement is the best yet?). All of these stuns happen on a single tile and it's important that this is a divine beam tile and it's also important to remember that the Wind Mushroom is building up a constant increased damage taken debuff. The longer they stay on this tile, the more damage they are taking and the goal is to hit them more than once with a Fire Curse for the tougher enemies.

You'll note how I listed that the Wind Explosive stun happens after the other stuns but how can that be?. This is very important. With the Hail of Fire upgrade we can manually decide where the Explosive tower hits and you want it to hit the tile AFTER the killbox. This is because of the explosion radius on the attacks of the tower. It extends naturally to about halfway through the tiles next to the impact zone. If we targeted the killbox, we'd actually be stunning enemies for 5 seconds before they entered the divine beam area. So the enemies get stunned by a few things entering the killbox, move to the halfway point of the killbox tile, then get stunned by the Wind Explosive tower.

Your main sources of damage are Fire Curse, the three Gatling towers and Fire Sniper. Fire Sniper effectively reduces the health of all enemies by 25% and is incredibly good. Fire Curse is going to be doing millions or billions of damage and is what's going to actually kill most things. Your three Gatling towers are going to be dealing incredible amounts of damage thanks to Titan Slayer. It's one of the few upgrades that scales with the enemies and will be increasing your Gatling tower damage with each wave. It's important to try and place the Fire and Wind Gatling towers in the center of your Lightning/Fire Divine beams for maximum flat damage. The multiplier of Titan Slayer wants a big flat base number.

As for what's going on in the killbox on the damage side of things: Three Cyclone towers lowering armor and increasing the damage enemies take by 100%, three Deathclouds increasing damage by 25%, Wind Mushroom stacking increased damage by 1%, three Curse towers lowering Health Regen and Armor which will cause enemies to take bonus damage from having negative armor, Fire Explosive lowering armor, Fire Laser lowering armor, the Fire tile increasing damage and lowering Health Regen, the Lightning tile chaining damage, the divine beam lowering armor and the Lightning Cyclone applying shock and increasing its own damage with each swing against a shocked elite.

The last thing to talk about are the modifiers you're going to be picking up at the end of each wave. There are four choices: Permanent enemy speed, permanent enemy Health Regen, permanent enemy Armor and permanent enemy Health.

We've discussed why giving them Health isn't a problem - Titan Slayer. Speed and Armor are a different beast. We want enemies to have as little armor as possible so our Curse towers can increase the damage the enemies receive by having negative Armor. Giving them armor a few times is okay if it's getting you something big like your last Hellement upgrade but I would hold off unless you absolutely need it. Health regen is laughable. Your Gatling towers will be doing 9999 many, many times a second and your Fire Curse will be chunking enemies for millions so 200 health regen or whatever a second means nothing. The real problem is enemy Speed. I'm still not exactly sure how it works but enemy speed is somehow tied to how long your stuns last. The internal speed of an enemy also seems to speed up how quickly they break out of being stunned. We want enemies to stay stunned as long as possible, so that's a big no. On wave 3 or 4 you're sometimes given the option of upgrading a Hellement for a very minor permanent enemy speed increase. This usually takes the enemies from a 5.5 to a 6 or 6.5. You can take this if you want but just know that way down the line this will eventually be what kills you.

So, in the order you want to take each modifier:
1 - Health Regen
2 - Health
3 - Armor

That's it. You're never taking speed so it isn't an option.

One last things before I go. Upon entering Endless Mode, the game will sometimes forget to give you your 1 Blessing at the end of each Boss Wave. If you enter Endless Mode, beat the very first encounter and then exit to menu and resume playing, you'll get your Blessings at the end of each Boss Wave like normal. This occasionally has the adverse effect of starting a Boss Wave when you re-enter and I can't explain it much better than seeing for yourself. It'll give you the option of taking any of the very first Boss Wave bonuses of 600 souls, 4 blessings or a random hellement. I don't know if this is somehow repeatable or abuseable but it's either you live with the bug of not getting blessings anymore or the bug of getting a random small reward in exchange for getting blessings again so you're damned if you do, damned if you don't.

There are a few other things I could talk about but I'm trying to keep this short. I can answer specific questions in the comments.