Risk of Rain 2

This will likely sound crazy early on, but this is actually easier (and very common) deep into runs. This is because your build will get so crazy that you'll be destroying everything, even bosses in one or two shots. This achievement is very easy to do as the Engineer. This is because the Engineer's two turrets inherit all of the items you got as well. So, you essentially have three people with the same items shooting the boss. Items that make this as easy as possible:
 

• Lens Maker's Glasses - 10 stacks will put you at 100% crit chance
• Armor-Piercing Rounds - each stack is +20% damage to bosses, so stack these endlessly
• Missile Mk1 - each stack is +300% damage for these missiles, so they quickly become extremely OP
• Brilliant Behemoth - this makes all your attacks explode, doing an extra 60% damage on top of the already high damage
• Missile Launcher (equipment) - this shoots out 12 homing missiles, and they do amazing damage. You don't even need to aim, so this is perfect for this achievement
• If you have the Missile Launcher, get Fuel Cells too. Each one adds another Missile Launcher charge, so you could potentially send out dozens of homing missiles at once

That's really all you need for this achievement. In the clip below, I complete the ingame challenge for beating a boss in under five seconds with the Engineer. This achievement gives you 15 seconds, so you can see how much time you have to spare. Also notice in the clip that I had to kill two Wandering Vagrant bosses for this challenge, and both still died in under five seconds. If you notice my items, I had the ten stacks of Lens Maker's Glasses, only two stacks of Armor-Piercing Rounds, six stacks of the Mk1, and just one Missile Launcher charge, but that was enough to annihilate both bosses nearly instantly.

https://www.xbox.com/play/media/CWVTSJeC2m