Child of Light

I found an easy way that requires minimal effort. As soon as Robert joins the party, fast travel to the Old Monastery from the Map of Lemuria. Once you're there, fly up to the ceiling, exiting the building. Fly to the left and down the first cliff you see. You should see a gargoyle looking monster sitting on the edge of a building. Perform a surprise attack on him and you should enter battle with 3 weak monsters. Now just use Robert's Rain Arrows ability on the monsters. They should all die instantly.

Now this achievement can easily be done right after you find Norah. Down below where you found her shoud be a Giant with what looks like a tree branch nest on his back. For this you will want to use Finn and Igniculus and depending on who spawns beside him (first time he had bird like creatures) using his spells to damage the 2 allies he has spawn with him and always keep igniculas on the giant to keep him slowed down even more. Once you delt with the rocks or birds you have to attack him with full force (I took him down with a fire spell and a slash), but if you get hit you can easily come back and fight him.

The video below of Clarky1005 shows a good spot to do this.

I recommand doing this after finishing the story!
I personally used Finn (fire spells) and Gen (stuns).
I killed one in turn one, and stunned another one. This way you can focus on the one that is still alive and not stunned. Kill him and make sure to cancel his attack when it is his turn.
Keep the third one stunned every turn to finish him off as last.
I hope this helped, good luck!

Easy way to do it for me was wen i got lvl 21 on rubella, go to the altar zone and try having various surprise encounters then whit rubella make an tumble all whit a fire oculi whit that level you can get Te achievement really easy.

I was unlucky and the achievement glitched for me. So i had to start over and i was looking for the fastest way to unlock this achievement. Here is a solution especially for people with the same glitch:

The "earliest" time to unlock this is at level 13 and it will take around 1 - 2 hours to reach this point. You can and should skip most of the enemys and level up quickly at boss fights. Just make sure to invest all your skillpoints in auroras starlight tree and your goal is starlight (all) at level 13... you can learn tumble (all) with rubella at the same time!

Now with level 13 use your map and teleport to the Old Monastery. Fly up and exit the building to the left and fly down. Now look for the gargoyle at the building and start a suprise attack. Now hope for 3 enemys and pick starlight all and tumble all. The starlight (all) attack should be strong enough to instant kill all enemys, but tumble all is your backup attack in case someone survive. Pling! Achievement unlocked.

I have to thank SilzzinBaC0n69 for the suggested enemy location in his solution.

Easy to get. Don't kill all the monsters in the early level's but carry on with the story. Once you get to Populi, a city populated by trading mice, you should have leveled up Aurora enough to have the Starlight Attack All skill. Go back to the early levels (I did it on Monastery) and find a creature. Make sure you are fighting 3 foes and then use Defend until you have enough time to use Starlight all, as it takes a long time to cast. With mist creatures it should kill them all three first time round. If not you will have weakened them enough to take them out with another attack.