please do something about these tank junglers

There is hardly any reason to pick a squishy jungler such as an assassin such as Khazix. Reason is that ganking any lane is super risky because of your level disadvantage. Ironically it's now assassins which are the power farmers and tanks which are the gankers.

Tank junglers build full tank and amass big KDAs because they can easily CC and set up the kill for their laner or secure it themself. Very difficult to 1v1 them because they will survive long enough for their laners to rotate. Plus their base damage and clear speed is very high so you have to really fuck up to fall behind. Whereas on assassin junglers "falling behind" is the same as "not being ahead".

I really like the positioning aspect of Khazix, having to pick your fights well and decide whether to leap/stealth for engage or disengage. He is a very fun champ. But nowhere near as easy in Gold solo Q as say, Skarner / Nunu / Amumu.

Nunu in particular both spam ganks AND powerclears. All while building tank. How do you deal with this?

Just played a game as Khazix vs Nunu that we lost at 20 min. Looking at the damage chart, we both did very little damage (around 3k). However with that low damage Nunu had a huge KDA (4/1/9) while I got just 1/2/1. He ganked around 4x more than me (ganked EVERY LANE FOUR + TIMES which is like a gank a minute) while still keeping up in farm (his 99 CS to my 109).