All tanks have to do is get a bunch of health and armor, i try having last whisper and bortk and void staff if im on kog maw. but a full build kog maw doesnt even scratch a darius with black cleaver and dead mans plate. how do i beat this tank meta?
you could land your q on darius shredding his armor and MR rendering him vulnerable to your bio-arcane barrage that deals a ton of damage if he stands in it for too long.
play an adc with some sort of mobility/cc, kogmaw is just easy prey
If you're kog, have a body guard. Otherwise just kite.
Use Attack-Move effectively
Work with your team and let them protect you.
Don't fight them 1v1
Don't stop attacking and kite.
ADCs actually naturally counter tanks, but only if you are able to sustain your DPS over a long enough period of time to wear the tank down.
even scratch a darius with black cleaver and dead mans plate. how do i beat this tank meta?
Darius is not a tank, he is a bruiser (and while DMP has good tank stats, both of those items are still bruiser, not tank items. There's a difference with some overlap). He has damage, not amazing cc. It's an important skill for an ADC player to know the difference, because the counterplay is different against the two, separate, classes.
It's not a tank meta. Far from it.
And to answer your question, it's about positioning, teamwork, and kiting. Kog'Maw isn't stellar at kiting when using his W, so either focus on poking with ult and other spells or time your W when your team can protect you. Now, that's advice if you're actually looking at advice, not looking to just complain and not actually do something. I don't mean that as an insult, it's what some people actually do here. If you're looking for advice, just use your range and stay protected by your team. Don't 1v1 with every ADC (and gods no with Kog'Maw, please don't. You're not playing Vayne with Koggy.
Anyway, instead of void staff, I would recommend Wit's End with Kog'Maw if you're going for an on-hit build. Rageblade, however, is a must. If you don't have rageblade, you shouldn't have void staff. Also, runaan's is so, so, so useful on him.
If you're having troubles with kiting, I recommend 
