Low elo is well known for players who don't have a whole lot of game knowledge. I'm pretty bad at this game, but I know how things work most of the time, and when I end up playing ranked that puts me in the high silver/low gold area. Knowledge is power, and simply knowing the game can cover for a lot of mistakes.
Instead of coming on here to make a QQ thread about an entire class of champion ruining what can only be ruined by yourself, take the blame you deserve and learn from these games..
If you're having issues with their healing buy grevious wounds. If you're having trouble with their damage get a defensive item or two. If you have trouble with their CC you're not playing against a Juggernaut, or you got massively outplayed.
Assassins are supposed to be weak to Juggernauts simply because Juggernauts are designed to absorb a decent amount of burst. Make sure you have resistance pen when fighting them, and if possible don't fight them alone. Their biggest weakness is kiting, so if you can bring slows and CC to the party when dealing with a Juggernaut.