Shadowrun Returns

I personally played the game as a mage, so always had access to offensive magic, but if you have chosen another path, you should be able to get this by hiring Mage class runners for jobs. I don't think that Conjured spirits count for this, it has to be from the magic you cast as your turn. As with most of the other achievements in this game, you can save right before killing some enemies then reload that save to grind out all the kills in about 5 minutes if you don't get this through natural gameplay.

My go to spells were Flamethrower, Acid Spray, Fireball, Manaball, Mana Bolt and Lightning Bolt (which I didn't use much). If you have any half decent firearm (shotguns are best for raw damage, but obviously fall off at range. I recommend carrying a Shotgun with an AR for long range work), you can hit the enemy with the gun then with Flamethrower 2/3 and it will kill basically any (meta)human sized target if they are flanked/you get a crit with one of them. You can cast Flamethrower and Mana Bolt every single turn if you want, which makes them excellent for this. Fireball and Manaball are basically magical grenades that come with the added bonus of DoT damage and are free, although have a cooldown.

Better versions of spells become available for use yourself, or will come on the runners you hire as you progress in the story. Standing on Ley Lines will boost the power or reduce the cooldowns for magic, but this isn't worth standing around getting shot for.