This is some terminology that will be used across the guide. If you don't understand something, feel free to come back here:

Terms:

• Capped resistances: Resistances on this game can only be effective up to 90%, so anything that gets you over this number on your character information means that this specific resistance is capped
  
• DPS: Damage Per Second. This is used to mean you're using a character to damage enemies, instead of using towers
  
• Hybrid: A character that is used to build towers with, but that has a set that gives them capped resistances
  
• Res: Resistance

Each character has a set of stats that you want to pay attention to. You can look at this by opening your Hero Information screen:

From left to right, top to bootom:
HP: Hero Health
Damage: Hero Damage
Speed: Hero Speed
Casting: Hero Casting Rate
AB1: The hero's first ability, this changes based on which hero you're using
AB2: The hero's first ability, this changes based on which hero you're using

When you're getting gear setup for upgrading, you'll always want to have in mind what stats are optimal for the character you're making a set for.

You can reference this guide, which has what weapons and pets are optimal for each character as well:

https://docs.google.com/spreadsheets/d/11v0h3LLUDYEkTxvkLVWUzYv8wXvjZ1cPTJJ-o1QqVDM/

But briefly, what is there is the following:

For upgraders you have 2 options:

• Jester, where you want Casting Rate with Ab2, getting the ab2 stat up to 3726. HP and Damage are nice but optional
  
• Small EV skins, where you just want Casting Rate, with all other stats being nice but optional

For boosting your towers, you currently only have the Monk. You'll want Ab1, with HP, Casting and Damage being nice optional additions

For boosting other heroes, you currently have 2 options:

• Countess: Ab1 them Da,age. with Casting and HP being nice sides
  
• Monk: Ab2 then Damage, with Casting and HP being nice sides

Countess does this really well, since she also gives other heroes a speed boost.
For monk, you only want to have a AB2 set if you're playing in a group, since Damage as a main stat ends up working better for solo content

For the rest of character not mentioned here, you'll want Damage then Ab2 with a nice but optional HP and Casting side

You always want to make sure you have at least 100 Speed, but that's relatively easy to do with Transcendent and above gear

This part is really important when playing any part of the games, since it also applies to builder gear.

When you have 4 of the same type of armor equipped, the stats on the armor will get a bonus based on the quality of the armor. You can see if your piece is getting the bonus by looking at your hero stat screen and seeing if all armor pieces have a blue glow on their item square. The numbers when you mouse over the gear will also have a blue color, instead of the normal white.


This is the extra stat bonus that you get based on which quality the piece is:
Mythical: 30%
Transcendent: 33%
Supreme: 36%
Ultimate: 40%

You can use pieces of different qualities with no problem, where the bonus will be applied based on the piece, not the whole set. You just need to remember the following:
The stat bonus only applies if you use all 4 pieces of Mythical quality and above OR all 4 pieces of Godly quality and below.

Something that changes once you get into Nightmare Mode and it's not really intuitive to see, is that your character's resistances get multiplier by 0.55, meaning you'll need to get 164%, instead of 90%, total resistance points on your character to be capped.

This means that just upgrading your armor resistances to 23% doesn't cut it anymore. You'll need to learn something that is also not very intuitive:

You can upgrade resistances an additional point after 23 every 10th upgrade level. This means that on the upgrade for level 10, 20, 30, ... you can use that upgrade to make the resistance on your piece get higher than you would assume is possible. If you look at the piece below, you can see that it's not possible to upgrade the generic resistance anymore, since it reached 23:
But if you just keep upgrading the piece, once you get to level 19, you'll be able to spend another point on generic resistance:

You can use this to your advantage to get your piece's resistances way higher than you would think it's possible. An easy and fast way to do this is by pressing Shift while clicking on the stat you want to upgrade, with Pro Mode turned on. The game will always stop on an upgrade where you can place another point on resistances.

Be extra careful as this doesn't happen when upgrading while pressing Control

OBS:
This extra upgrade is only possible on Mythical, Transcendent, Supreme and Ultimate pieces, so this doesn't work for the early game with godly lower qualities.

This is something you should always pay extra attention to when upgrading pieces that have a low number of upgrades or highly negative resistances.

You'll always want to upgrade whichever resistance is the highest first, to use your extra possible upgrades as soon as possible. For example, on the piece you've been seeing on the left, if you upgrade anything but fire resistance first, you'll only get to use your bonus resistance levels from level 30 or 40. If you have low upgrades, this can be the difference between getting the piece to cap resistances or not.

If you want some extra examples, you can take a look at this video where I show how to upgrade some pieces:
https://youtu.be/aHBuMIEWXQQ

Due to the low set bonus and max upgrades that mythical gear can have, you'll need to prioritise some resistances over others.

You won't be using mythical gear for too long, so you can just use this part as a base for getting into early Nightmare dps content.

Since all mobs do generic damage, you want to ALWAYS get the generic resistance upgraded first on mythical armor, that way you won't be getting extra damage dealt to you by EVERY mob in the game.
For the elemental resistances, my personal priority is Fire > Lightning > Poison.

Just be aware that every percentage you're missing from 90 means you take 10% extra damage from that element


This is what a really good mythical set will end up looking like:

For the individual pieces, you'll want to upgrade each resistance to 31% with no set bonus, while paying attention to the priority mentioned before. This is an example mythical dps piece:
As you can see, I upgraded both fire and electric resistance to 31%, while getting poison and lightning to 28%

For transcendent gear, depending on the number of upgrades you have you'll need to prioritise specific resistances just like mythical pieces.

On top of that, due to rounding, for transcendent pieces you won't be able to have the resistances stop at 41%, they'll either be at 40% or 42% with the set bonus. Because of this, you'll usually want to alternate the resistances between two paired pieces, like shown below:
As you can see, the boots have 30% generic and fire resistance while the gloves have 31% without set bonus. This way, you get 82% between the 2 pieces and don't end up wasting points that can go into the other resistances.

With high enough upgrades, you can get capped resistances on trans pieces, but by the time you get good enough pieces like this you'll most likely already be using supreme or ultimate dps sets

For Supreme and Ultimate pieces, due to how many upgrades they end up getting and their respective stat bonus, you don't need to worry about resistance priority anymore.
You just need to be sure to upgrade your Supreme piece resistances to 30% before set bonus

For ultimate pieces, like on supreme, you don't need to realistically worry about which resistance to focus on. Due to the set bonus for Ultimate gear being 40%, you only need to upgrade your pieces to 29% on each resistance, which can be done with only 200-220 upgrades if you're lucky with the spawns.

This is a way to upgrade your pieces that saves you one upgrade per piece.

ONLY DO THIS IF YOU ABSOLUTELY WON'T TRADE THE PIECES AWAY.
Pieces upgraded like this are considered worthless when trading

You'll want to upgrade one resistance on each piece to 31%, while having all other resistances at 28%. The reason you'll save 4 levels while doing this is due to how rounding works on Dungeon Defenders. 31*1.4 is 43.4, and that rounds up to 44%.

The best way to understand this is by visualising it, so I attached a set upgraded this way

So you got yourself a nice Ultimate or Ultimate++ armor piece that only has 2 or 3 resistances and want to use it? You can do it, as long as you get yourself another piece of the same type with enough upgrades.

For upgrades on both pieces, you'll want to have at least 350, but usually 380 is the number you'll want to aim for.

Example pieces:


On this case, the helmet doesn't have the fire resistance, so you'll want to use a complementary piece that has all 4 resistances, in this case the boots.

You'll want to have the sum of the resistances in both pieces be 82% with the set bonus. The easiest way to do this, is by upgrading all resistances on the piece that only has 3 to 35% without the set bonus, while on the other piece, you'll upgrade the resistances present on the 3 res piece to 23%, while upgrading the one that's missing all the way to 58%.


Upgrading this way, means that when you have the set bonus, the missing resistance will be 82% (82 + 0), while the other 3 resistances will be at 82% as well (59+33). That brings the resistance average from the pieces to 41%


For 2 resistance pieces, you do the same process while making the sum of resistances with set bonus be 123%. An example would be the following:


The only issue with using this method is that your pieces will need to have higher average upgrades, in the range of 410-430, but if you get a super good DPS piece with only 2 resistances, you can still use it

Here's a video with the whole upgrade process, just in case:
https://youtu.be/z8Tq4DBEp04