Vossler is one of the hardest bosses so far, it's been a while since stuff like Brynhilda and Seifer/Rajin/Fujin so would find it hard to rank it. I would probably put it at the hardest (other than the broken Parade Float) because you need good character relics to master (or even beat) it.
Some tips:
If you don't have your own Sentinel Grimore/Stoneskin II then don't cast it until the adds are dead and Vossler is at ~95-90%, remember to rebuff it ~45-40%. The fight is long and you need three casts for the entire fight plus adds.
If the adds die before their third turn you don't need to bring Shellga, there is some leeway here with luck and slow giving you extra turns. Took me 4 turns.
Bring Full Break and Power Breakdown/Steal Power. I didn't start casting Full Break until the adds were dead but Power Breakdown needs to be up the entire fight.
Be ready to hit Dispel as soon as the phase changes, this happens about four times. If you have time you can dispel Reflect too to waste a boss turn.
You'll probably need an AoE Heal to heal up after the adds, I cast a second at the end too but it probably did more harm than good.
Bring DPS, I had one each of Double Hit and Tempo Flurry bouncing off Cloud, with Cloud having his own Tempo Flurry.
Remember to slow at least one add and hit Vossler with Black Magic, I used a VII Wizard Bracelet to have Cloud cast the shared soul break black magic attack on it.
Egg Paladin Cecil to 80 and give him max defence, his Draw Fire relic helps too as it buffs defence. Keep him at 100% health during the last half of the battle, even Quadstrike can only do 190 x 4 damage to him properly set up.
Don't bring mages their low health is a liability and of course my White Mage died at 1%, my first failed attempt had two White Mages and two AoE Heals - you can't heal up the damage as it's so focused on one character - fight is about mitigation.
80 Cloud - Retaliate/Tempo Flurry, Wizard Bracelet SB, Dragoon's Devotion RM
80 Paladin Cecil - Boost/Dispel, Sentinel SB, Double Hit RM
80 Wakka - Full Break/Armor Break, Status Reels SB, Ace Striker RM
65 Balthier - Tempo Flurry/Steal Power, Self Sacrifice RM
65 Eiko - Curaga/Protega, Emerald Light SB, Dr. Mog's Teachings RM
Geared Cloud for Attack, everyone else for Defence. Actually used a lot of 3++ synergy stuff and had a 6++ Sword for Cloud and a 5* synergy Shield for Cecil. Mastered it without RNG being an issue, it does take skill so you will S/L to learn the fight.