Risk of Rain 2

As the description says, you have two options for unlocking this achievement. I'll first briefly discuss the first option, escaping the Planetarium, as it's the more challenging route (in my opinion) since it requires you to beat the game on an alternate path, and fight some bosses that are tough. After that, I go into more detail on the Simulacrum option, because it's an absolute cakewalk with the right setup.

Either way, once you purchase and install this DLC, you need to enable it in the game, on the right side of the character selection screen. This enables the new characters to choose from, new items in game, and new maps to show up in rotation. Now, for the Planetarium option for this achievement, you need to actually reach the Planetarium, which requires a bunch of work. First, you need to reach a stage with a Newt Alter you've donated a Lunar Coin to, so that, after completing the teleporter event, a blue portal opens up. Enter it to enter the Bazaar Beyond Time. In the Bazaar Beyond Time, there is a hidden portal under the standard blue portal to leave, and you need to jump down to it to find it. This Null Portal, which is accessible only once per run, takes you to the Void Fields, a hidden realm. Once there, you need to breach all nine cells on this map. Doing so opens a Void Portal which takes you to a new hidden realm, Void Locus. In this hidden realm, you need to clear all Deep Void Signals, which opens the Deep Void Portal. Interacting with that portal takes you to the Planetarium. This area features a unique boss fight that includes multiple platforming challenges where you need to run, jump, dash, etc., to avoid an encroaching Void Fog that will gradually kill you. On top of that, you need to fight a tough boss across multiple phases. Once you do all that, you can finally leave the Planetarium and unlock this achievement. Needless to say, this is the much harder way of unlocking this achievement.

The far easier way is by playing Simulacrum, accessible from the Alternate Game Modes option on the main menu. Note that this will likely take close to an hour just because of waiting for enemies to spawn, but it is not hard at all. To make this as easy as possible, first unlock the Artifact of Command and the Artifact of Sacrifice, as described in the base game roadmap. You then should also unlock the Engineer if you haven't completed the base game yet (the Engineer is unlocked upon completing the achievement Engineering Perfection (30G) for finishing a cumulative total of 30 stages across all runs of the game). That's all you need. Make sure you set the difficulty to Drizzle, and turn on both of those Artifacts, then go ahead and start up the Simulacrum. This is a horde mode with infinite waves, but don't worry, it isn't bad at all. Here's how it works:
 

• Every wave will consist of small groups of enemies spawning and attacking you. There is a built-in lull in spawning to give you time to kill enemies
• Every wave ending with 5 (i.e. 5, 15, 25, etc.) there will be boss fight (except on Wave 5), and then when the wave is done, the little purple NPC that controls the safe area you're in will get up and move to a new location. You need to follow along and stay in the safe space, but don't worry, it moves slow and doesn't take any crazy routes
• Every tenth wave (10, 20, 30, etc.), there will be a ton more enemies, and they will step up in difficulty from then on. They will also get an item (one of the ones you can get, such as damage reduction, increased movement, etc.) to make them stronger. After beating each tenth wave, you'll have to hop in a portal to continue in a new location, but the same setup applies

So basically, survive five waves, beat a boss, follow the NPC to the next area, survive five waves including a hectic one at the end, then hop in the portal to move on to the next set of ten waves.

Now on to the setup. You want to be the Engineer so you can drop both of your turrets in the safe zone and just let them do all the work. You can shoot at enemies in the first few waves to help out, but eventually you won't need to do anything. The most important part of making this a cakewalk is the Artifact of Sacrifice. This absolutely breaks this game mode. I believe this game mode was designed to be such that, every time you finish a wave, you're given an item to help make you a bit stronger. However, with the Artifact of Sacrifice, enemies will be dropping items in addition to the one per wave you're supposed to get. So, instead of getting one item per wave, you'll be getting easily 4-6. It's not huge in the beginning, but it very quickly snowballs out of control as you get to later waves. Coupling this with the Artifact of Command that lets you choose whatever items you want, you can make absurd builds. To put it in perspective, by the time I reached Wave 50, I had 20 Lens Maker's Glasses (+10% chance to crit), 30 Crowbars (+75% damage to enemies above 90% health), the Brilliant Behemoth (all attacks explode and do +60% damage), many stacks of Predatory Instincts (+5% chance to crit, and critical attacks increase fire rate by 12% each), along with tons of defensive items just to be safe. However, with this setup, I literally wasn't even seeing enemies, nor was I even seeing bosses. They would spawn and my turrets would instantly kill them in a single shot, before I could even locate them. All I was doing was running around picking up the item drops while my turrets just obliterated everything instantly. Here is a video showing it in action:

Now for some more detail on what items to choose to get this build going.

White Items

• Get a Bustling Fungus right away so your turrets keep themselves alive. One is enough. No need for more
• Start stacking Lens Maker's Glasses immediately after that. Once you have nine of them, start getting Crowbars. Note that this item stacks linearly, so nine of these, at +10% crit each, means 90% chance to crit. The remaining 10% comes from Predatory Instincts and Harvester's Scythe in green below
• There is no end to the improvements you can get from Crowbars, so one you're doing critical hits basically every time from the Lens Maker's Glasses, it's time to start amassing these, which will allow your turrets to one-shot everything. Like I said, I had around 30 by the end. Get as many as you want
• Tougher Times and Personal Shield Generator are good to grab a few of, just to be safe, especially in the beginning when enemies and bosses will actually be on screen for some amount of time. I would say three or four of each is fine
• By Wave 30 or 35, grab a couple Goat Hooves to make movement better, only because there are some explosions you'll need to avoid in Wave 40 and the Engineer is a bit on the slower side

Green Items

• Get Predatory Instincts right away, and don't hesitate to build up four or five of these
• Get one Harvester's Scythe to max out your crit chance at 100% (after the nine Lens Maker's Glasses and one Predatory Instincts)
• Get a couple Missile Launchers. They do insane damage
• If you have Old Guillotine unlocked (I didn't) you can take a few to help with bosses and elites later, but I didn't use it and I still one-shot everything

Red Items

• Brilliant Behemoth is your first priority
• Dio's Best Friend is your second
• Unstable Tesla Coil is your third

Equipment

• I know people swear by the Preon Accumulator, but I much prefer the Missile Launcher because you don't need to aim. When a boss spawns, just press  and let it destroy them

That's really all you need. Feel free to pepper in anything else you want, such as more movement, an extra jump or two, etc., to make yourself feel safer, but once you get the damage up from Glasses and Crowbars, nothing else will matter much.

There are two other things to mention:
 

• In Wave 40, Void Reavers will spawn (at least, that was the only wave they spawned for me during my 50-wave run). They will still die instantly, but if you haven't encountered them in the base game yet, know that when they die, they create a purple sphere of implosion, and anything caught in it dies instantly, including yourself no matter what items you have. You have time to run away from their implosion, but because my turrets were killing them instantly, I didn't have time to see where they spawned to get away from them. Just stay on the move, and pay attention to your turrets, because there's a pretty decent chance a Void Reaver implosion will wipe out a turret, Make sure you set a new one to replace it, and keep moving to avoid any yourself
• For Wave 48, I'm not sure if this was randomized or what, but Artifact of Glass activated for that one wave, which puts you (and your turrets) at only 10% health but 500% damage. One hit will kill you here, but by Wave 48 your build should already be in overdrive and killing everything instantly, so you stand no chance of getting hit. Just know that you'll stay at 10% health when Wave 49 starts until you heal, so stand next to one of your turrets with Bustling Fungus to heal back to full very fast

That's it. A lot of information, and it'll take some time, but it's quite easy to do, and you'll never feel more powerful than by the end of this achievement.