Does this achievement involve killing all seven of them on the same character, or can it be done across several?
Same character only. 
You didn’t really think it would be that easy did you? 
Well thanks for confirming lol time to keep grinding
It’s actually less grinding to kill them all with one char. Took me 10 min to get all of the new achievements.
I assume you meant to say 10 hours, or 100 minutes?
You can’t kill all 7 Magis in one single dungeon run, you’d need at least 4 runs, and you can’t possibly do one run in less than 3 minutes. Even if you restart the session each time you killed 2 Magis and use GrimInternals to teleport to the key gate it takes much longer than 10 minutes to encounter all 7 Magis.
Finding and killing Rigg’s Associates takes longer than 10 minutes too unless you’re extremely lucky.
All of the above in 10 mins is a tiny bit hyperbolic don’t you think? 
Pretty sure I got it in only 2 runs using my fastest farmer with 2 low CD mobility skills and killing every boss in under 3-5 sec including Morgoneth
But maybe it’s a different achievement, now that I think about it. Will check after work tonight.
That’s mathematically impossible as there are only 2 Magi spawns per run. Even if Morgoneth’s summon counts (which I believe does not) you’d still need at least 3 runs.
It took me 6 runs to get the achievement, and I think that was lucky. If RNGesus had a bad day, I could well have encountered the same 2 magis over and over again every run.
Yep I think you just confused the achievements. No worries mate, I didn’t bring this up to slander you, just to give people no wrong ideas when they look up the info about these achievements and find this thread.
At one game only have two different magi,but the all have seven, it’s just need u kill the all seven so many times,i have finished.