So far I've never lost an A rank due to not having enough KOs or passing time limit.
In fact I feel like I'm getting stuff done much faster.
Plus with the help of said Fairy.
Very easy to get between 200-500 KOs in a single cast if you're in a well populated area as the Fairy knocks out unrendered enemies.
I only managed that 829 because it was a Rack Up KOs stage which has tons of monsters around. :p
The only time I really feel it slowing me down is if I've got an NPC ally in a keep as allies will take up some of the rendering spots making less enemies appear but on most maps that's not much of an issue. Also it only seems to be keeps that Allies take render priority. I usually find wandering enemies like Raid Captains and such are always visible over grunt allies.
And I haven't mentioned this but the Fairy magic attack is just a single button press that casts straight away like a special attack.
Fairies use up 60%-100% of the magic gauge depending on what you've got them wearing, 2 same-element clothing that reduces cost means you only need 60% of the gauge to cast, and only need to pick up one more small jar to cast again if you were at 100%.
And this is the range of the attack. (Keep sizes for scale.)

Oh I almost forgot this part.
Adventure map isn't always "You must use this character."
There are some where you have to but there are many where is says restricted, but then lets you bring another warrior or 2 of your choice, which you can freely switch between in battle.
In above picture with fairy magic range, it was a Darunia restricted, but let me take a second fighter. I took Tingle because I love his attacks where he floats into the air and explodes a massive bomb. :p
While I'm on about Fairies and stuff, here's what my main fairy looks like.

I do have Demon Lord clothes for her, but no matching headwear so she's got Royal on for now. :p
I renamed her Shayla because her default name was Snorre, which sounds too much like Snore. Shayla sounds much better IMO.