First of all, he resisted two attacks from Whitebeard, bisento + quake bubble, and he shot Whitebeard, and killed him with his crew. Even Akainu was unable to continue to attack Whitebeard after quake bubble + AoE quake attack.
Then he fought against Garp + Sengoku with no problem.
Later, he defeated Marco + Jozu + Vista. Which is something even Akainu couldn't do, when Akainu fought against them, there was no Jozu and he had half of the marine army, Kizaru and Aokiji. Teach had no Admiral level fighter at the moment (maybe Shiryu ?) and he defeated a more powerful pirate group by Diamond Jozu's addition.
Now he is trying to recruit Ex-Admiral Aokiji into his crew. Just how powerful is Current Teach ?