The Best There Is

This achievement only needs you to beat 15 out of the possible 24 challenge mode fights. There is no achievement for beating all 24.

Since there's no similar comment, I thought I'd list the fights from easiest to hardest, based on my opinion as a 720 warlock, just in case people are looking for an idea on which fights to skip. Obviously, this is not a consensus, it is just my opinion:

A Seagull - Easy - Stay close and rotate about 90 degrees around the boss for Blinding Peck. That's it.
Ixx - Easy - Stay close to the centre of the hitbox and just run forward when it casts. That's it.
Millie Watt - Easy - Stand in the pools when she's casting the Discombobumorphanator. Other than that, not much to it.
Deeken - Easy - Just dodge walls and dodge frontals.
Thog Hammerspace - Easy - Run away when he casts Booster Bonk. Dodge to the side when he casts Da Fif Hammer
Nibbleh - Easy - Kite in a big circle. Don't stop. That's it.
Johnny Awesome - Easy - Don't stand in green circles and make sure red arrow from Powershot hits the dog.
Blind Hero - Easy - Keep distance/stay behind and nuke. Run away when he casts Blind Cleave.
Proboskus - Easy - Interrupt Torrent, move out of circles. Do not let it melee you after it Berserks.
Vian - Easy - Make him face a wall and strafe the Fire Lines. Dodge the fireballs moving back and forth.
Fran & Riddoh - Easy - Move a few yards when Throw Dynamite is cast. Move out of the mines when the net wears off. Kill Riddoh first or he enrages.
Sanoriak - Easy - Stay in the centre of the room. Interrupt Pyroblast and quickly move out of Firewall. He also uses Heated Weapon, which hit me for around 16M damage, but not sure if this has a cast time or is just random.
T440 Robot - Easy - Just dance in and out of melee range every 20 seconds or so. Feel free to use the affix that makes you cast whilst moving for this one.

Dominika - Medium - Adds are the main threat. Stack on the boss and they will eventually walk into melee and can be AoE'd down. If you lack AoE, I can imagine this being a harder one.
Vishas - Medium - If you can kite, this is really easy. If you can't, I can see this being a harder one as he hits pretty hard.
Umguloxx - Medium - If you can survive being wailed on, or can perma-slow, this is probably easier, if you can't, it hits hard. Avoid the Stampede charge.
Blackmange - Medium - Just an awareness fight. Keep an eye on the lines and move to a cannon that's just fired. Will be easier for some.
Dark Summoner - Medium - Just proper execution of mechanics with a DPS check. Will be easier for some.
Leona - Medium - Adds really hit hard and there's more as fight goes on. If you can kite/slow it should be easier.
Kirrawk - Medium - Hit's relatively hard and can sometimes be tricky to aim twisters into the boss. Interrupt Storm Cloud as this really hurts.

Yikkan Izu - Hard - Do not kill the adds or boss enrages. More adds will spawn over time so damage ramps. Interrupt the disorientate.
Fjoll - Hard - Hits hard with quite a few annoying mechanics for casters.
Mama Stormstout - Hard - Seems pure RNG where the safe zone spawns. If it spawns right where the wave spawns you have no choice but to just stand on it and wait for the wave. Recommend skipping.
Akama - Hard - Hits like an absolute truck. Skip if possible.

Feel free to share own opinions too, these are just mine.

Good luck all.

This achievement gives progress if someone you are in party with kills the bosses.
It makes things go alot faster

Does anyone know if there is a hidden FOS achievement for doing all 24?