The achievements guide for Fight Night Champion from XxBlueSpade2xX & KFZ Scrubs (props to them) is very nice and all, but it lacks depth regarding the rival gyms achievements. So, here it goes, hope if helps!
This one will test you patience. Not because it's hard or anything, but rather because of the odd way gyms work and the timing involved. If you are lucky you can get it the day you set up to do it, if you are not, you may have to way up to 15 days for a chance.
There are two ways to get this, with a friend (aka the easy way) or with randoms. Anyway, make sure you have 3 created fighters (one light, one middle and one heavyweight).
With a friend:
First, go to XBOX LIVE menu, then GYMS. Now, if you are not currently in any gym, just use the SEARCH GYMS option. If you are in a gym, then go to MY GYM, then SEARCH GYMS. Leave all the search option alone, except for ACCEPTING JOIN REQUEST (set it to YES) and PUBLIC GYMS ONLY (should also be YES) and then choose START SEARCH.
You will be presented with a list of GYMS. Now, scroll down through the list and keep an eye on the right panel where it says CURRENT RIVAL. It can be NONE or it can be a name of a gym. If there's a name, memorize or write down what is the gym in the list on the left (let's call it gym A) and what is the gym that is its current rival (let's be original and call it gym B), hit
to go back to search options and this time around search for the name of gym B. Check it out, and see if this gym is also accepting join requests and is a public gym. If it is, congratulations, you are halfway there, if it's not, search again for public open gyms and repeat the process. You join gym A, and tell your partner to join gym B.
Note 1: if you search for rival gyms, and can't find a single one, fret not, the game is not broken (much...). That's just the way it is, you may have to wait up to 10 days for the rivalry assignments unfortunately , keep checking back, someday you'll find a pair of gyms. If you are curious, you can always join any gym, just to check the timer for next rivalry assignment. But beware that when you quit a gym you may have to wait 24 hours cooldown to join other one.
Note 2: At the point where you try to join a gym, if you any of you are currently in a gym, you'll have to quit it to be able to join other gyms. If that's the case, you may get the "You have to wait 24 hours to join other gym" message. There's no way around it, you'll just have to wait.
OK, now you are both rivals to each other. Any of you can go to the gym menu, pick GYM RIVALS, then CURRENT RIVALS, then press
(FIND RIVAL OPPONENTS). You will be presented with a list of players of the rival gym, just pick your friend and press
, then select Invite to Fight. Select lightweight/middleweight/heavyweight, finish the fight and both will get the achievement, no matter who wins.
Note 3: at this point, when you try to invite your friend you may get an error message saying the you can't invite the rival because the rival season has not started yet, or something similar. If that's the case, than you'll have to wait whatever the timer in the top left of the screen is showing. Again, there's no way around it. Just wait whatever days the game says, then go online and do it.
Do pretty much the same to the point you are in a gym that is currently rival to some other gym. Go through the list of rival players, and message a guy asking if he's up for a rival fight. That can take a while, I tried a couple of times without luck.
The process is the same for all weight classes. Get all three in one go, if you have a chance! Any questions let me know.
Many thanks to Be Certified for doing this with me!