malphite is the best lategame tank, since only him and rammus can hold off ad carries for some time, but rammus can't pass the tank line like malphite can, if u go into lategame, it's really hard to lose with malphite tank.
Try using w -> e -> activate randuins, all champions hit by full combo will have their attack speed decreased by 86% to 95%, depending on randuins proc, so a champion with 1.8 AS will be attacking at a speed of 0,25 to 0,09. Ad carries will be rendered useless for about 3 to 4.5 seconds (because of knock ultimate's knock up), they should be death by now, if not, they'd still take at least more 10 seconds to kill you with auto attacks, leaving time for your ad carry to drop about 5k damage and time for your ap carry to do at least 2 full combos.
People may say you don't have mr, and u'll die to magic carry, but with 4k hp, they'd have to throw everything it at you, and if they do, your team already won. And that's beacause of malphite ultimate, that make him unpokable by squishys late game.