Risk of Rain 2

This is it - the hardest achievement in the game. It was actually made much tougher with updates over the years. Hidden Realms, such as the Bazaar Beyond Time, the Celestial Portal, etc., used to count as stages too, so you didn't actually need to play through 20 stages. Now, though, you need to play through 20 stages, and this achievement will unlock after finishing your fourth visit to Sky Meadow, after going through the teleporter.

This is a really tough achievement because, even though the timer makes difficulty go up over time, progressing in stages makes the enemy level take step changes up even faster. The good news is that enemy difficulty maxes out around Stage 17 or so. The bad news is that enemies will be level 99 by then, so it will be absolute chaos. That being said, if you've made it that far, chances are your build is solid enough that you won't notice a huge step up in enemy difficulty. By then, you'll need to have a solid enough build in place to ensure that enemies die instantly and/or you take virtually no damage. Otherwise, single hits from some enemies can one-shot you. First, I will go through the general gameplay strategy I used, then I'll go through the build I used. Feel free to use whatever strategy and character you are most comfortable with, but I do feel that mine is pretty user friendly to just about any skill level.

First of all, before even attempting this achievement, there are a number of things you'll want to have unlocked first, to make this as easy as possible:
 

• The Engineer - this character is unlocked the same time you unlock Engineering Perfection (30G) 
• Artifact of Command and Artifact of Sacrifice - this eliminates all RNG from your build, so you absolutely want these both unlocked and active. Refer to the roadmap for doing so if you somehow haven't already
• N'ukuhana's Opinion - this red item unlocks upon unlocking Her Concepts (30G) 
• Old Guillotine - this green item doesn't have an achievement, just an ingame challenge, and it's for killing 500 elite enemies cumulatively. Work on Deicide (30G) first to likely unlock this item during that process
• Harvester's Scythe - unlocks when you unlock Prismatically Aligned (30G) 
• Predatory Instincts - unlocks when you unlock Rapidfire (30G) 
• Dio's Best Friend - unlocks when you unlock The Lone Survivor (50G) 
• Unstable Tesla Coil - unlocks when you unlock Macho (20G) 

So basically, do everything else first. 

Now onto how I played every stage. I would always beeline right to the teleporter, starting from the first stage. Look for the fireflies and just head right to them. Once at the teleporter, I would set up my turrets in good spots for the first few stages, spread apart on higher ground. In later stages (after Stage 8 or so) I would always put them down so their Bustling Fungus healing zones overlapped each other, so they could heal each other in addition to themselves. I didn't grind enemies on any stages. I would first let my turrets kill off any enemies chasing after me, because I'd want no distractions when the bosses spawn. Then, I'd activate the teleporter and focus on the bosses. With them dead, kill enemies until the charge is finished. I found that this would result in me taking 2-3 minutes per stage, but still getting 5-6 items per stage from the enemies during the teleporter charge. If you have a lot of enemies still around you before activating the teleporter, go ahead and kill them before doing the teleporter. Like I said in the roadmap and earlier here: don't let the timer pressure you into rushing unnecessarily. Enemy difficulty steps up after each stage anyway, so the timer eventually doesn't really matter anyway. It's much better to get a few more items than to finish a stage a minute or two faster. 

White Items

• Lens Maker's Glasses - 9 of them for 90% chance to crit (the other 10% will come from Harvester's Scythe and Predatory Instincts in the green section below)
• Bustling Fungus - at least 5 of them to improve healing speed and radius. IF you get N'ukahana's Opinion, stack these higher, because that'll increase damage it does
• Armor-Piercing Rounds - get at least 5 so you're doing double damage to bosses, but there's no upper limit here
• Soldier's Syringe - I wouldn't start getting these until a bit later in the run, but getting a few will make your turrets jump from enemy to enemy much quicker, getting kills very fast
• Energy Drink/Paul's Goat Hoof - get a few of these total, just so you have better movement, both for finding the teleporter faster but also to help avoid enemies and their attacks in later stages
• Tougher Times - get at least 10, to block at least half of all attacks, but there is no upper limit here. Once you have everything else above, stack this one with white items nonstop. By the time I reached Stage 20, I had 50 of these and essentially was invincible

Green Items

• Hopoo Feather - get 1 to help with mobility and since you have no other way to avoid fall damage as the Engineer, but feel free to add another one or two
• Wax Quail - get 1, just because that burst of speed forward is really the old dodge you'll have as an Engineer, and it helps with quick movement
• Harvester's Scythe - 1 is fine, for 5% more chance to crit. Don't waste more than that
• Predatory Instincts - 4 or 5 is good, the first giving 5% more chance to crit, and the rest allowing your turrets (and you) to fire at double speed, basically
• Old Guillotine - a solid 10 is ideal, but they aren't as important until you get to stage 10 and beyond, so focus on others first
• Ukelele - get two or three or so of these, so anything coming near your turrets gets zapped, and this procs the missiles below
• Fuel Cell - I got a few of these, because each adds an equipment charge. I think I eventually had 10 uses of the Missile Launcher, which is just ridiculous, and there was virtually no cooldown
• Missile Mk1 - stack these as high as you want, because your turret shots and Ukelele will be proccing missiles like crazy, and they will do insane damage the more stacks you have

Red Items

• N'kuhana's Opinion - this pairs with the Bustling Fungus that your turrets will be continuously healing with, causing them to constantly send out skulls with this item that do huge damage
• Brilliant Behemoth - all attacks explode for an extra 60% damage. Great for your turret fire
• Dio's Best Friend - gives you an extra life, but your turrets get an extra life too every time you place one
• Unstable Tesla Coil - constant powerful lightning strikes from you AND from your turrets
• Rejuvenation Rack - doubles healing, meaning N'kuhana's Opinion fires twice as often, effectively doubling its damage output. I wouldn't recommend getting this unless you already have that item though

Yellow (Boss) Items

• Shatterspleen - makes crits cause Bleed, and bleeding enemies explode. Since your crit will be at 100% from the white and green items, every hit will cause Bleed and every enemy will explode. The explosion radius is huge. This item is probably the biggest game changer for later stage mobs. Getting this practically guarantees your success

Equipment

• Missile Launcher - I know many people swear by the Preon Accumulator, but you need to aim that and it travels really slow. The Missile Launcher shoots 12 homing missiles, it requires no aim, and no missiles go to waste (they will fly around until they find a target). This is perfect for both bosses and the chaos of mobs, because you can just fire this off whenever while you concentrate on moving around to avoid attacks or on attacking specific enemies

I should note that, I realize this list above makes it sound like you need crazy luck to get all of these things and this many stacks. However, with the Artifact of Command you can choose exactly what you want. I recommend focusing on damage output and mobility the first handful of stages, but by the time you loop for the first time, start getting defensive stuff (Tougher Times) to be safe, because enemies start being able to kill you far faster. Also, I know I got somewhat lucky, but you want to keep your eyes open for Scavenger enemies. These are big bulky enemies with huge backpacks. They are quite the tanks, with more health than most bosses, but killing them results in them dropping their backpack. Interact with it to get like 6-8 items, green and white. This is a massive boost in items for your inventory. I killed three Scavengers on my successful run, and it helped tremendously. They won't start spawning until later stages, but it really helps fill out your build.

Lastly, I want to say that, during my successful run, I didn't get a single red item. It seems insane to not get one, especially considering I played to Stage 24 before obliterating, but it just goes to show that you don't need everything above to be perfect in order to be successful. I did get Shatterspleen though, and in my opinion, it's far better for this build than any red items.

Just for fun, here are a couple clips of my character in action. I don't think it's possible for me to die, because enemies and bosses, even level 99 ones past Stage 20, were dying before I could see them, and I had 50 stacks of Tougher Times, so I don't think any attacks would even damage me, if somehow an enemy didn't immediately die from my turrets. Also, no, these clips are not sped up!

https://www.xbox.com/play/media/gfecYFELzB

https://www.xbox.com/play/media/qP2kTcKNvi