So after about 2 years attempting this achievement (on & off), I finally got it done!
For some reason I found Tag more difficult than the 100 Single Wins, so thought I would share some tips... (I will try to keep it short but no promises) :)
I am assuming that most people reading this will use Hayate as the primary fighter. There are lots of good tips and video guides on here so make sure you are fully confident using him.
You do not need to master ALL of his moves but when the opponents start blocking you need to be able to mix up some basic high / low kicks & punches - without panicking!
(On this note - if they start blocking don't be afraid to tag out immediately! - Switching fighter is sometimes enough to confuse the AI & break their defence).
Also, always try to tag before your health gets to half-way. This will help keep your fighters balanced.
Now on to my main tip - the thing that finally got me through this:
***** Use Ein as your tag partner *****
(You can unlock him by completing story mode as Hitomi - also, he was the only additional character I had unlocked).
Obviously if you are good with someone else then feel free to use them but I had tried various different partners - Christie, Ryu Hyabusa, Hitomi & Kasumi.
None of these got me past 75 wins - Then I got to 100 wins in about 10-15 tries using Ein.
Ein is good because he is powerful and relatively simple to use (much like Hayate). Originally I wanted him as a back-up, but by the end I think he was even quicker at dispatching opponents than Hayate. : )
I only ever used about 6 of his moves & still kicked ass! Here they are:
1. QCF + Punch: Basic power punch - same as Hayate.
2. Back + Punch + Kick (Hold to charge): Do this whenever you opponent is on the floor / getting up. If it does not send them flying, it will Stun them - setting up for 1 of 2 moves...
3. Punch + Kick (Elbow strike) - follow quickly with another Punch. (This is a good basic move which sends them flying & rarely gets countered).
4. Punch, Punch, (Hold) Punch. This is what u want to use on the ropes. I think you can actually do it twice in succession for a total of 6 hits. (Get used to holding the final Punch as this will greatly increase the damage).
5. Forward, Forward (hold to run) - Kick, Kick. Kick.
(This is the same as Hayate's running triple kick - but not as powerful). Best used after a power punch to close-down the distance.
6. Almost forgot this one! - Forward + Kick, Kick.
(A simple front Kick that pushes them back - they almost never block / counter). Good for keeping them away or setting up the next attack.
Finally, 2 tips to save your sanity:
1. Try aiming for 50 wins and then take a break... Remember that, (in theory) the next 50 should be EXACTLY the same! I stopped at 50, both times when I got 100 wins in Single & Tag - it just helps to break it up.
2. I suggest playing little and often - like once or twice a day. Any more than this just builds frustration. I started playing once when I got home from work each night & I had it by the end of the week...
So that's about it!....I hope I explained things correctly. Please let me know if this helped you in any way.
Keep at it guys & good luck!