Uh, it forces it to switch which lets your team set up on it. It also prevents a sweep if Ranku is the last pokemon.
Great, my stall team forced it out. Now what? As I note above, I'm yet to find something that can repeatedly take it's attacks while phazing it over and over. You can't just Phaze it once and say 'lol it'll be their last Poke' because that's not how it works. In order to force it to be their last Pokemon, you have to deal with their entire team while consistently Phazing it repeatedly. This is almost impossible in reality, because it means you need a Pokemon which can constantly switch into it while either taking attack or letting it get to +1, meaning the attack you take while Phazing will hurt even more.
Use Taunt on your stall team. Use Perish Song on your stall team. Use Wobbuffet on your stall team. Use Spiritomb on your stall team. There are plenty of options for stall to deal with it.
Taunt doesn't beat it, it just forces it to kill you. It also only lasts 3 turns, meaning you have to be dang bulky and kill it quickly or you'll end up using Taunt a crapton of times, which is just giving it more opportunities to rip holes in your team.
And even if there weren't, why would "beating stall" mean that something should be banned from all of OU? By this logic we should ban Clefable, Cradily, Terakion, Tyranitar, Wobbuffet, etc etc. I'm going to go out on a limb here but it sounds like your stall team needs work if you instantly lose when Ranku comes out (as you seem to be implying).
No, because Clefable etc. do not beat stall on their own. Stall has reasonable ways to deal with them. In fact, I've never had a serious problem with any of them. I've never had a serious problem with anything apart from Rank that I couldn't fix by tweaking a few Pokemon. I found my team was Dragonite weak, so I replaced Salamence with Gyarados. I found issues with Doryuzuu, so I made changes to help deal with it. This is part of teambuilding, but in ~50 hours of testing I haven't found a single thing which beats Rank while contributing usefully to the team.
It's not our job to tell you how to play. It's your job to come up with new strategies (even though we already have given you plenty). If you are going to quit instead of innovate, then you are precisely the kind of user that we don't want.
I'm not quitting because I don't want to innovate. I'd be quitting because I didn't believe OverUsed was an enjoyable metagame. Also, you say you have given me plenty, but you are yet to give me a good one. If there was a Pokemon that forced say every offensive team to run an otherwise useless or completely outclassed Pokemon, people would clamour for bans, but because it is Stall that is given the treatment, random offense players are coming along screaming 'not broken' because their chosen playstyle does not have trouble with it.
You literally summed up the reason why we have a rating requirement for voters. "Unless someone tells me how to win, I quit". Classic.
That isn't what I said. I said that unless appropriate changes were made to make OverUsed an enjoyable metagame, I would no longer want to play competetive pokemon. Maybe your team doesn't have too much trouble with Rankurusu. Well guess what. I could make a team that doesn't have trouble with Mewtwo, but nobody is arguing that it is not broken.