According to the wiki,

Originally posted by Nuclear Throne Wiki:

"The I.D.P.D. (Inter-Dimensional Police Department) is a group of enemies set on killing the player when they reach a certain level of power. Gameplay-wise, they are designed to be the equivalent of the player, but using tech to augment their abilities rather than mutations."

The I.D.P.D. are not mutants - as a result, they do not drop rads (experience) upon death, they can only drop ammo and health if you're damaged. When they enter a level, a siren noise accompanied by a pair of, or more, blue portals will appear, along with troops. They can hurt you and other enemies, but not themselves. The policemen and women have unique abilities, special guns that fire faster blue bullets, and only spawn under certain circumstances, such as:

• When you damage a Proto Statue or fully charge it.
  
• When you open a special blue I.D.P.D. Ammo Chest.
  
• If you take a crown from the crown vault, they will spawn as you enter the next level. (If you start off the game with an unlocked crown, they will immediately spawn in 1-1.)
  
• When you enter the Palace, they have a random chance of spawning. (Except in 7-3).
  
• Lil' Hunter has the ability to summon 2 I.D.P.D. at will.
  
• Upon looping, I.D.P.D. will always spawn in the Campfire area. (0-1).
  
• When playing as Rogue, the I.D.P.D. will spawn when you kill approximately half of the enemies in the level, when you kill everything in the level, and can spawn like this in every level except for 7-3.
  
• After looping the game, the I.D.P.D. will spawn randomly in every area excluding the Crown Vaults and 7-3, and two I.D.P.D. portals will appear whenever you kill a bunch of enemies in the level. The post-loop I.D.P.D. are unique, stronger variants of pre-loop I.D.P.D. When playing as Rogue, in loop, the "half-level" and "finished level" I.D.P.D. spawn in addition to the post-loop ones.
  
  
  
  

Now that you understand who the I.D.P.D. are, and when they can appear, it's time to learn about each individual enemy and what they can do.

Starting with...

Grunt


Interesting note about the grenades; you can use a melee attack to bounce the grenade back at the thrower, saving yourself and the cover you're hiding in.

Inspector


Like Grunts, Inspectors are generally not much of a threat if you play smart and stay at range, giving yourself time to dodge their attacks.

Shielder


Shielders are serious business, and the toughest of the pre-loop I.D.P.D. enemies. Take your time when fighting one, as they can reflect your bullets right back at you, take lots of hits, and dish out plenty of damage in short bursts. Interesting note, though; using an energy weapon, such as a Laser Pistol, shoots straight through any Shielder's shield. The same goes for melee weapons. Use this to your advantage when they aren't able to shoot back by hitting them through their shield.

After fighting your way through the Throne and looping the game, you've got a whole new set of I.D.P.D. after you. Immediately after defeating the Throne, and arriving at the Campfire area (0-1), several pre-loop I.D.P.D. will spawn, and after subsequent loops, the post-loop I.D.P.D. described here will start to spawn in the Campfire area.

We begin with...

I.D.P.D. Van


Vans are dangerous run-killers. If a van touches you, you instantly die, regardless of how much health you have. The only preventative measure you can take against this is to take the Strong Spirit mutation. When a van enters a level, you should immediately turn your attention to it, as the I.D.P.D. that it unloads are much more of a threat than the other enemies. A common strategy is to grab an Energy Sword/Hammer, follow behind the van as it enters the level, and kill each troop immediately as they spawn when the van unloads. Killing the van isn't required, however it is a nice investment to sink your ammo into it in hopes of getting more ammo or health back. Stay back from the van when you attack it, though, as the explosions can surely kill you if you aren't careful.

Elite Grunt


Elite Grunts are tough. With their new Throne Butt-like ability, they can fly very fast across the level and hit you with near perfect accuracy. In addition to being tough to hit, they have about as much HP as a regular Shielder, making them hard to kill. The rocket he fires is an instant kill unless you're Crystal with Rhino Skin/Fortress Ultra, or have the Strong Spirit mutation. The rocket also tends to blow up cover that you're behind, so hiding isn't always the best option. The easiest way to kill him is by waiting for him to stand completely still as he prepares to fire his gun.

Elite Inspector


Elite Inspectors, in my opinion, are the least scary of the post-loop I.D.P.D. troops. While their telekinesis is strong and can throw off your dodging, you should be absolutely fine as long as you keep your distance. Don't sit idly by and let an Elite Inspector charge you, as their attacks cut deep. The best time to shoot them with bullets is when they aren't actively using telekinesis, so your bullets connect instead of bending around them. Bolt and Energy weapons also aren't affected by the telekinesis.

Elite Shielder


Elite Shielders are by far the toughest post-loop common I.D.P.D. troops. They are the beefiest of the Elites, have the deadliest weapons, and has arguably the best ability of the three. When you see an Elite Shielder, immediately focus on him, as he is likely going to be the largest threat. When taking cover or running away from one of his Plasma Minigun blasts, be careful where you take cover - plasma projectiles explode after contact with anything, including walls - it's entirely possible that the plasma explosion will reach through the wall and kill you. The only saving grace about the Elite Shielder is that the Plasma Minigun projectiles travel much slower than other I.D.P.D. bullets, but they still aren't very slow. Elite Shielders can teleport, but this ability is currently bugged; he won't teleport right next to you, though he can teleport about and create sounds in places where he isn't. Take utmost care when fighting Elite Shielders, as small mistakes will cost you your run. Don't panic, take your time, and shoot him while he's vulnurable.

You now know about every single I.D.P.D. troop, both pre and post-loop. Except for two very special I.D.P.D. that warrant their own section dedicated to them due to their uniqueness.

Beginning with...

I.D.P.D. Freak


I have never personally encountered I.D.P.D. Freaks, as I've never reached Loop 3. (Furthest I've been is 3-2 L2.) I have no idea how these enemies behave, nor do I have any strategies for beating them. If you've gotten this far into the game, though, you're likely good enough to deal with them somehow. And if you get this far, good luck.

Next, we have the quicker, more menacing...

Lil' Hunter


Lil' Hunter is a force to be reckoned with - and without the proper setup with regards to mutations or weapons, he will quickly kill you. He is designed to be a tougher I.D.P.D. troop, but not as tough as the head honcho. He has about the same amount of HP as Big Bandit, but has several more ways of attacking. The streams of bullets travel incredibly quickly, and are difficult to dodge, whereas the cluster of bouncy bullets is near impossible to dodge in enclosed spaces. The best option is to take cover before popping out to engage him, but if you stay too far away and cower behind a wall for too long, Lil' Hunter will fly up into the sky, and slam back down into the ground where the player is standing, exactly like when he spawns in the beginning of the level. This, combined with his ability to spawn I.D.P.D. troops, makes him a beefy, mobile opponent that punishes your own use of cover. To take him down, you have to have hard hitting weapons, like a Heavy Crossbow or Assault Slugger - and melee weapons help deal with his flurries of bullet patterns.

And last, but not least, we have the notorious...

Captain

I won't be going into super in-depth information about the Captain, or how to beat her. She's an intense secret boss fight, and if you wanna read something other than just basic stuff about the Captain/motive or strategies to beat her, read this lovely guide: http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=642196648

That being said, here's the Captain's stats:




The Captain is an intense, super-difficult boss fight. You need the perfect mix of both survivability and weapon-specific mutations, in addition to the perfect weapons and skills, to defeat her. Mutations such as Rhino Skin, Strong Spirit, Back Muscle, Rabbit Paw, and Recycle Gland are almost required to take her down. If you're going to fight the Captain, be sure to start preparing your build at the start of your run. If you happen to run out of ammo during the fight, and don't have Rabbit Paw, you're merely delaying the inevitable. I don't think that you can use Gamma Guts on the Captain, because I believe she deals contact damage in the same way that she does when she charges at you.

Little Fun Fact about the Captain, she speaks broken Finnish! Her opening lines translate to "Dirty mutant!" and during the fight she'll yell "Fist!" when charging you, referencing the fact that her robo-morph suit is literally shaped like a fist. If you die, her lines translate to "I am the law! Be gone!", which I think is pretty neat.

Rogue, though she isn't an enemy after defecting from the police force, is technically an I.D.P.D. trooper in her own right - she is a futuristic, combat-trained human - just like every other member in the ranks of the I.D.P.D.



Unfortunately, Rogue is too complex of a character to cover in one guide section alone. Her playstyle caters to experienced players looking for a challenge, though with clever thinking and practice, you can go as far into the lategame as top-tier characters - like Y.V., Horror, and Melting - with ease.

Linked below is a guide that will cover, in-depth, Rogue as a character, how to adapt to her playstyle and adopt an appropriate mindset to help overcome her more challenging aspects (i.e. Crown Unlocks, the B-Skin Gauntlet, etc). Feel free to read it - I hope it will help you understand Rogue and assist in your endeavors to get 100% completion.

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=655992966&preview=true

Congratulations!

You're now an expert about each I.D.P.D. troop.

You have reached a maximum killing proficiency with regards to "da popo".

Yung Venuz would be proud.