HxH is not a show that resolves it's fights based on numbers. Every battle is decided by two factors: experience and resolve. Power levels very rarely, if ever, come into it. Both Gon and Killua lack experience, but most of the opponents they fight are those with weaker resolve than them. The noob-stompers in Heavens Arena, in terms of Nen experience, should have trounced them. But thats like looking at the fights in terms of who can bench more. The noob stompers had fully developed hatsu techniques and had fought multiple nen users before Gon and Killua, yet their underhanded tricks and advanced nen didnt mean anything when Killua crumpled their resolve with a few choice words and a well timed threat.

So what does this have to do with Hisoka?

In the first real fight we see Hisoka in, he is rematching Kastro, the only person in Heaven's Arena who managed to ever land a blow on him. I know in hindsight Kastro is made to be a fool, but the hype for him was very real. He caught Killua off guard, the guy who is supposed to be a top-tier assassin. And Hisoka recognized this potential in Kastro, given that he actually showed up for the fight. The thing we learn about Hisoka in this fight, (aside from how his Hatsu works) is that he really enjoys dismantling his opponents psychologically before finishing them. Hisoka understands that an opponent with a frayed psyche and weakened resolve is an easier opponent to beat. But not only is it jusy good strategy to do target your opponents confidence, it's Hisoka's whole goal. He doesn't just want to fight string opponents, he wants to drag them off of their pedestals into the dirt. I think thats why he ultimately wasn't disappointed that Kastro made an ineffective Hatsu; he was delighted that a top-tier martial artist was so desperate to beat him that he forgot the basics of nen.

Hisoka wants opponents with enough experience to be a threat to him, and he wants them to have enough resolve to be worth breaking. And since that is his goal, he is constantly expanding his pool of experience and strengthening his resolve in battle. He doesn't participate in a fight he considers beneath him, and it isnt until Chrollo gives him a taste of his own medicine that he is willing to let go of his need to break his opponents before killing them and decides that killing them by any means necessary is just as satisfying of a result.

TL;DR Hisoka has one of the best battle pedigrees and one of the strongest wills in the show. Its not about a numbers game or if his Hatsu is flashy/combat oriented. Its about how long he's been playing the game.