Fable Anniversary

Seems to be a whole load of confusion regards this achievement but it's quite simple when you understand what's going on.

Level 1 summoning spell will work fine,all leveling it up does is increase it's duration before it will despawn.

When you first get the spell you will summon a wasp which is the lowest enemy in the game.

How the spell works is when the wasp lands the killing blow on an enemy that is higher than it your summon then becomes that creature your spell will never downgrade.

So Wasp kills Bandit in Great Woods Entrance you now have a Bandit summon, Bandit kills Balverine in Darkwoods you now summon a Balverine. If you watch when your creature kills a higher level enemy it instantly morphs into the new summon and that's what pops the achievement.

All that needs establishing now is what is the lowest creature you can kill to pop the achievement. I personally had my bandit kill a minion but they are near the end game. In the other solutions comments it does mention someone getting it from a Balverine so as early as Dark Wood could be a possibility or it might have been a Balverine from later on in Knothole Glade.

Received a number of confirmations now that you need to upgrade it to a Balverine.

Hope this helps.

Edit:- I've looked into the spell a bit further in old guides & found this http://fable.wikia.com/wiki/Summon

Observed sequence of upgrades: Wasp(starting creature)->Stronger Wasps/Insects->Bandits->Undead->Bandit Chief->Stronger Bandits->Stronger Undead->Balverines->Stronger Bandit Chief->White Balverine->Minion Warrior->Minion Spellcaster(strongest)

Edit 2:- Important: Blue beetles have issues attacking an enemy for you and you may be stuck with it.

Here is how i did it and i think it's the fastest way to get it done.

First go to the hero's guild, get sumonner level 1 and go in the cullis gate. Now teleport to barrow fields and head to dark weir. When the loading is done, you will have a balverine in front of you. Just hit the balverine until it's almost dead and spawn your wasp and let it kill the balverine.

Achievement unlocked !

If you kill the balverine instead of your wasp, just go back to barrow field and back to darkwood weir to spawn again the balverine.

This'll pop as soon as you are able to summon a Balverine. You only need the level one spell so start by summoning a wasp. Use your Wasp to get the killing blow on a higher level creature e.g a Hobbe. You'll notice your summoned creature is now a Hobbe. Keep killing higher level enemies in this way, using your current summon to kill a tougher creature, and work your way up to killing a Balverine, at which point it should pop the achievement.
I went Wasp - Hobbe - Bandit - Bandit Boss - Balverine.

Buy the Summon spell from the Heroes' Guild using General XP, Will XP or a combo of both. The upgraded spell will make this easier but it's not necessary (as far as I know).

Use the spell in combat. The wasp that spawns has to kill an enemy for the achievement to unlock as it turns into whatever it manages to kill. I'm not sure what constitutes as a "powerful creature" but I personally used it against some bandits in Witchwood. I used a max Summon spell, hit a bandit a couple of times to weaken him, then allowed my summoned wasp to finish him off.

i did it this way with a Level 1 summoning spell :-

1, wasp kills bandit
2, bandit kills bandit chief,
3, bandit chief kills balverine.

Achievement unlocked :)

I just unlocked this by having the wasp kill an assassin waiting for me outside Knothole Glade. The first and only kill I have made with the summon.

Just blocked loads as he attacked me, I hit him two times and just kept summoning the wasp, eventually the assassin died and pop went the achievement.

Now I got a pet assassin. Yay.

You can get this rather fast while doing the main quest Rescue the Archaeologist.

After talking to a character at Grey Manor, your task is to go to Bowerstone Jail. When you arrive there, a few enemies will spawn on the bridge. Take care of them until one remains and then cast your level 1 summon while blocking. You will never get hit and just stand there and wait for your wasp to slowly take the enemy down. You don't have to slowly level your summon this way, because this one enemy popped the achievement straight away for me. You can also do that directly with a balverine, but those can knock you down while you only have to block while idleing here.
Rolling away from you wasp, so that the enemy is between your summon and yourself makes it survive a little longer, because the enemies does not accidentily hit the wasp while swinging for you. You might also count how many hits the first enemies can take before they get killed, so that you can prepare the last one a little bit. If you kill all enemies, some more will spawn up the hill