that's ridiculous, one jutsu doesnt put you at Kage level
One jutsu separates Asuma from Minato. FTG.
This is assuming Asuma HAS an A-Rank technique that could match Minato having Rasengan.
The jutsu you use actually DEFINE your skill level. I don't see any other STANDARD Genin (No Naruto or Sasuke please) using A-rank jutsu other than Konohamaru.
Kakashi beats Mei and possibly Gaara. If he was anything short of Kage level, he'd be mincemeat in his current fight. You're downplaying him too much. Not only does he have Kamui, but he has powerful Lightning jutsu such as Raikiri, and is able to use an A-rank Suiton technique.
He did just fight off Kekkai Genkai users, the Seven Mist Swordsmen, then go up against Tailed Beasts. And now he's fighting Obito after using Kamui HOW many times in a non-Uchiha body? He's definitely Kage level. He defeated skilled Jonin Zabuza with the handicap of having to protect a non-shinobi and three Genin (although they saved him, keep this into account. Plus Haku had to save Zabuza later). Then he easily defeated Zabuza later in a sticky situation.
Zabuza was not a fodder ninja. He was a "high Jonin". And Kakashi defeated him fairly easily. Zabuza's abilities were actually a BAD match up for Kakashi because he could no longer use his primary fighting style, his Sharingan.