He is fine.

Heal reduction
Block debuffs
Cc

If you had a more mature account you would just one shot him.

Short answer: Ma'Shalled and Tornado do not need a nerf. Salad is a strong champ, but one that can be countered. Immunity - which you mentioned - is one. 

Having a high resist champion that goes before the rest of your champs and can cleanse, or one that's faster than Salad that can put up block debuffs. A high resist team in general, or perhaps going faster than Salad to either CC or kill him. 

Some or all of these things might be a bit difficult for now. Eventually you will get to a point where Ma'Shalled becomes a lot more trivial of a fight. 

It's funny - back in the day he used to be a pretty clear sign to avoid a team. Now, on the other hand, he's a pretty clear sign to fight them.

Does he bring value elsewhere in the game? Unquestionably. But in arena, he does exactly nothing. Cardinal teams ignore his abilities entirely. Go-second teams ignore them entirely. And speed nuke teams? Well - they ignore them entirely too, since there's rarely room to fit him into any decent setup.

Ya i just got Ma-Shalled myself. My account is a bit like OP's one. So in my case, can Ma'Shalled make my top12 for tag arena, in many cases, sure, why not. But at the same time, I am fully aware that eventually, he will drop out of that group or even the reserve group, because if he can do 'well', he's not a team carry. Lots of defenses just laugh at my Ma'shalled LOL