Fable II

Level up one of the direct attack spells to level 5; Slow Time (though you can use it first to make things easier) and Raise Dead won't work. Then you need to find a group of 5 bandits that are willing to attack you with melee. There are a few places that they will do this but it will probably be the hardest part to do. Once they are on their way to attack you, start charging the spell to its max.
You will need to be careful doing this as you will most likely take hits while charging. Getting more health with skills or potions may be necessary.

While all of the other solutions are correct, I actually discovered that I was able to get this achievement by leveling up Inferno to level 4 (instead of the suggested level 5 by the other solutions). I then went to the Bandit Coast, charged up my spells to level 4, and on my first try I got 7 people with one spell.

I got this achievement by using a level 5 lightning spell. After hitting the assassins with a normal melee attach a few times to weaken them, then using a level 1 force push to buy some time to charge up to the level 5 strike.

A good place to find enough badies is the Brightwood Tower, after the main quest is complete and before the Brightwood Tower quest

level up your lightning spell to level 5 and go 2 the crucible. theres a couple levels where theres nothing but bandits. just charge up your power and and wait till theres 5 around you and let go. you may take hits while charging the attack so be sure to buy lots of potion before entering.

Managed to do this with shock level 4 during the Treasure Island of Doom side quest without really trying. Here's how, wait until you get to the room with several waves of 'dead' pirates - use 'summon the dead' at level 1 to distract them then charge up a shock radius attack to level 4 and you should get it.

Level up any of the spells that damage enemies. (Lightning, Fireball spells) to level 5. This won't be too hard, if you want to you can simply buy a tonne of magic and general XP potions. Then fast travel to Giles Farm. Just outside the entrance to the farm there are around 6 or 7 bandits. Charge up your chosen spell (surround obviously) and let loose. Easy as.

I know there are a few solutions to this, but I wanted to add my own since I had a spot of trouble with this one. If you haven't gotten this achievement by the time you get to Bloodstone, because you cant find 5 easy to kill humans (who are not villagers), you can unlock it during the quest "Treasure Island of Doom!"

I don't want to spoil anything, but I had a LV 5 Inferno spell.. and met up with the first group of enemies in the quest, which is like.. 3 waves of 6 - 8 humans each wave ( I didn't actually count them.. but there's tons of opportunity in a little room).

You're welcome.

You can actually get this achievement and possibly other similar achievements like it, early on in the game and I'll tell you how!

1. Kill a load of enemies to get some starting experience points or if you have a splitscreen buddy and they don't mind, they can help you if they have a high level in their abilities then it will be easier on you or even go the potion shop in Albion to buy as many potions as possible for the second player or just do it solo to make life easier on yourself and have the second player join as a dummy character, (FYI, it will say something like, "You'll need to be signed into to save gold and experience", ignore this and don't sign in, just continue, (FYI, this will not work online as far as I'm aware due to you not being able to transfer your abilities over to someone else or them to you, has to be splitscreen I think, sorry)

2. Once you have a bit of experience, spend it on abilities to gain higher levels or get the second player to either discard all of their abilities as they will have the same levels and abilities as you if they join in splitscreen. If you have higher abilities such as level 3 onwards, it will work faster and you'll gain more, mind you, I only had starting levels in my abilities such as Level one's and two's so it doesn't matter if you have higher abilities or not, the only difference is you'll gain more experience when you discard them with player two.

3. Discard every ability with a second controller till everything is down to nothing or till you wanna stop, (remember discarding more means more experience, FYI) then press start and quit, it should say something along the lines of, "Do you want to transfer your cash and experience to the main player when you quit", Hit yes, the second player will disappear, you will then have all of their experience that they had when they discarded their abilities and gold if they had any.

4. You can then repeat the process as many times as possible till you either max out every ability like I did or until you wanna stop at a certain point. Rinse and repeat, let player two join by pressing start on a second controller once again, best done solo though, also, when doing this every time, keep the gold and experience shares on 50% then once in game, discard everything once again. The good thing about this is, once you level up your main character more and more levels, the second player will have exactly the same abilities as you, meaning, say if you have done this till one or more of your main character's abilities are at level 4 or 5, when your second player joins in, they will be able to discard the level 4 or 5 abilities and you'll get a whole lot more experience back once you quit the second player out of the game and transfer the gained experience back to your main character, enjoy!