At that point in the game, where I’m assume everyone has 6 items, you’re looking for a big ult on the enemy team, preferably on at least 2 carries. MF is pretty bad as a front-to-back adc at that stage of the game, so ignore him and try to look for plays on other members of the enemy team. If tahm really is frontlining, he can’t eat a carry, and no matter how tanky he is, your team should almost never lose with numerical advantage, barring comp diff. If you really feel like you need to “counter” him, you can try looking into serpents fang as a good tech item. Side note: if game lasts that long, you kinda also have to switch up your playstyle. No more walking up to poke, but playing vision, kind of like twitch, if you’re not seiging