The way I understand it, it's not that he's the "absolute best". His original 50x buffs are super-short, each one lasts for barely a second. It's just that he's worked with this team for so long that he figured out their habits and short-comings, and he's learned to time his casts in that exact one second. It's also why he was adamant about training with the mage on upper floors, he wanted to perfect his timing for her casts so that there wouldn't be any faulty casts during more dangerous runs.Honestly.... this is a little bit dumb.
His advantage was supposed to be that he was a Jack of all Trades who was powerful in everything, even if not the best.
In pure buffs, he should be inferior... his advantage should be how well he works in battle since he can use every job...
But he's not a jack of all trades, he's a super best in the world master...
This approach doesn't actually make him better than other more specialized supports, but it makes him a much better fit for their specific party because both his support and his more generalist skills were tailored to them specifically.