assassins are supposed to rapidly deal the highest single target damage in the game, on par with bursters but generally in a lower amount of time with Mobility instead of cc, then they run out of steam, at which point their output is outdone by sustain damage fighters and ADCs.
If you're having issues with assassins might I suggest zhonyas, steraks, and GA in your builds sooner?? Not all three but they really do help against burst
unless you're playing tanks then, since I don't really play tanks, I'd really suggest that you try out some alternative builds to deal with assassins
Yeah even non meta crap, because non-meta is for the average game, so if what you consider to be meta isn't working then it's probably not the most effective tactics available (Meta)
Sometimes you gotta sacrifice that sweet damage for defense, it feels like less damage but assuming you have a main, you'll learn mechanics that will help to make up the difference.