Deep Rock Galactic

Dreadnoughts can be found in Elimination missions, Deep Dives and can sometimes randomly spawn in a level (very rare). They are large creatures, and you will know when one is near as their health bar will be on the screen.

The Dreadnought's armour is impenetrable, but their weakness is shooting them in the back. However, they have a 'shield' which can also be seen in the health bar. Once the shield is down, continue shooting them in the back (glowing area) to deal permanent damage. The shield will recharge once a third of their health is lost or after a certain time.

The driller can deal damage with the flamethrower or cryo cannon without shooting its weak spot. I would recommend the flamethrower over the cryo cannon though. Also explosives are useful against them (especially the Driller's explosives).

UPDATE: There are now three possible Dreadnought spawns from an Dreadnought egg. These are the original Dreadnought, Dreadnought Hiveguard and Dreadnought Twins.

The Dreadnought Hiveguard is immune to attack and will spawn several Sentinels (depending on the amount of players in game) that you must kill before it reveals three weak spots; one on its front and two on the sides (with health bars). Destroying these will reveal its weak spot at the back for a certain amount of time (similar to the original Dreadnought). Repeat this process until it is defeated.

The Dreadnought Twins (Arbalest and Lacerator) are not immune to damage and you can begin attacking them immediately. It is useful to mark these as they frequently bury themselves and reappear somewhere else. Every so often after they have taken some damage, they will heal each other and average out their health bars (they are immune to damage during this). I find the best strategy is to try and damage them equally as they don't usually heal, although this can be difficult. They have some strong area attacks so be careful. Thanks to RockTomorrow for confirming that killing both Twins counts as 1 kill towards the achievement.