For this achievement you can do it legit by completing the missions as looking for the other achievements. Or you can grind it with a semi-idle method:
First make sure you already have your companions unlocked, and they are at a good level. (level 10 as I did)
Then choose a level D mission that you are sure they can beat the boss 100% without you. Class B, C have bosses with more health, and that means, more time to defeat them, and the XP is not worth the time wasted on it, for example:
Class C > Destructive EarthTime: 120-180 seconds (companions' level 12)
Reward: 16XP points each time
So always look for the Class D missions:
Class D > Steel NightmareTime: 30-45 seconds (companions' level 11)
Reward: 14XP points each time
The other D missions:
Class D > The ButcherTime: 30-40 seconds (companions' level 11)
Reward: 13XP points each time
Class D > beholder, Time: 30-40 seconds (companions' level 12)
Reward: 12XP points each time
Class D > Imperial GuardianTime: 20-35 seconds (companions' level 12)
Reward: 11XP points each time
Class D > Lava GuardianTime: 20-35 seconds (companions' level 12)
Reward: 10XP points each time
Please note that if you choose to participate as well in the fights the time will drop around 10-15 seconds. (and it will reduce the overall time as each level your team increases)
Doing this process eventually you will unlock.
And if you start earlier
And if you haven't just yet: