So while Zabuza fights, he combines Hidden Mist Jutsu, Silent Killing, and Water Clones, in one attack. If he wanted to, he can be hidden in the mist, use SK to locate his target, have Water Clones attack the target while performing Giant Vortex Jutsu, so 4 skills/jutsus simultaneously. Against two targets, he could even have the mist up, have Water Clones attack 1 opponent using SK while having a Water Clone using Water Prison on the other, use SK to locate 1 of the targets then use Giant Vortex Jutsu, while hidden in the mist to attack them. 5 skills/jutsus used at one time, that is OP
Not only that, but if he somehow can't move or his arms don't have mobility, if he can move 1 of his hands, he can make 1 handsign with 1 hand and perform Water Clone Jutsu, which originally requires 2 hands, which further shows his skill. Then, as shown against Team 7, use the Water Clone to use Hidden Mist Jutsu and use it to Silent Kill opponents. If you think about it, if he was up against a shit load of Chunin, he could just use a Water Clone which can use Hidden Mist Jutsu to Silent Kill them all, just a simple Water Clone can assassinate a whole group of Chuunin, possibly even Jounin like Shizune, Baki, and Kurenai.
If you actually think about it, Zabuza is really strong and is a high jounin inbetween/in the middle of A and S rank.