EA SPORTS UFC 3

This achievement is a grind and there aren't many easy ways around this at this point.

You get 20xp for every win you get in Ultimate Team and 10xp for every loss so you progress no matter what after every fight.

I mainly did my fights in Single Player Ultimate Team Championships.

The first ten levels for each belt go by quickly depending on if you win a lot. Each level requires more and more XP to level up until you get the next belt (which are at level 50/51 and 100/101).

My final count of XP was the following:
Single player UT Championships:
900 wins x 20xp = 18,000xp
81 losses x 10xp = 810xp

Online UT championships:
12 wins x 20xp = 240xp
16 losses x 10xp = 160xp

So you'll need approximately 19,210xp (give or take a few XP, could be 19,200xp exactly?) to reach level 150 in Ultimate Team.

Here are some tips that helped me win more than I lost (in Single Player championships):

-Pick a good fighter who can take other fighters down easily. I used Demetrious Johnson from the Champions edition. They should have a high grapple rating.
-If you are using someone like Demetrious Johnson, you're probably going to face A LOT of Conor McGregor fighters.
-When you start the fight, open up with strikes to the head. They'll probably be blocking a lot but it's better to do this before you go for a takedown. Otherwise, they'll deny your takedown more often than not. This is also dependent on the fighter you're facing.

-Try to go for single leg takedowns. Double leg takedowns can be dangerous if you're facing a good fighter who is very good at submissions. Avoid full guard from the top at all costs unless you're comfortable with it.
-Once you have a single leg takedown, wait for the AI to try and transition. You'll be in half guard. If the AI puts their hand on your hip, hold right trigger and flick the stick as much as possible in the direction of where the AI's hand is (i.e., if they are using their right hand on your hip, hold right trigger and flick the stick to the right).
If the AI tries to climb over to take your back, hold right trigger and flick the stick as much as possible in the direction they are climbing (i.e., if the AI is climbing on you from the left side, hold right trigger and flick the stick to the left). You'll know if you did it right if you stay in half guard after the animations end.
The AI will try this 1-2 times usually.

-After the AI attempts transitions in half guard, transition to side control. Sometimes you'll get denied depending on the fighter you're facing, but if you transition right after the AI finishes theirs, you'll get there as the AI's stamina will be drained a bit.
-Once you're in side control, throw 2-3 strikes. If you end up being in side saddle, transition to side control and wait. The AI will try and transition out of side control. They will do 1 of 3 things (depending on what side you're on):
If the AI pushes their arm/hand into your face, hold right trigger and flick the right stick up as much as possible.
If the AI scoots backwards, hold right trigger and flick the right stick in the direction they are going (i.e., if you are on the right side of the AI and they scoot backwards, hold right trigger and flick the right stick to the right).
If the AI rolls on their shoulder, hold right trigger and flick the right stick in the direction they are rolling (i.e., if you are on the right side of the AI, they may attempt to roll on their left shoulder, hold right trigger and flick the right stick left).

-The AI will usually do 1-2 attempts at transitions in side control. Sometimes more depending on how low/high their stamina is. Once they stop transitioning, throw some more strikes and stay ready for more transitions.

-Once the AI has their stamina depleted in side control, go ham and throw strikes til they are knocked out.

-This mainly works for Conor McGregor and some other fighters. Fighters that I decided to not try against included Tony Ferguson, Jussier Formiga, BJ Penn, and Dooho Choi. They went for submissions a lot and it's near impossible to escape especially when you get higher in UT levels.

-Sometimes you'll get denied takedowns and be in a sprawl position. Do your best to get to half guard and go for a back side transition to gain back the dominant ground. If you want, you can also deplete the AI's stamina by denying transitions in Back Side. Just hold right trigger and flick the right stick in the direction they are moving.

-If you're denied too many takedowns, your stamina will be very low (thanks to the latest patch). At that point, give up and lose. You're probably not coming back and trying to is a waste of time against the AI.

-Don't quit the match though. I don't think the game gives you XP if you quit out the match. It's better to just let the AI beat you and take the 10xp.

-If you're having trouble denying transitions, practice, practice, practice. I would suggest the tutorials and practice mode to get better.

If you vote negative, please comment why. I can refine this solution with any other tips you guys come up with. The above is what worked for me and may work for you.

TL;DR - Practice the game and play it. This is a time consuming achievement.

Edit: Still works as of 11/25/22 :)
Get this done before servers go down!

There is a glitch in ultimate team to make the game think every fight is knockout mode.

This can allow you to win fights in under 30 seconds for your win XP. This obviously being much quicker for grinding out the monstrous amount of XP needed for level 150 than actually playing 15 minute bouts.

First, load up ultimate team
Click solo challenges on the far left
Click lightweight
Click challenge #1 out of the list of 10
Load it up
As soon as you have control of your character, quit out
Then, play as many single player championships as you like

The solo fights will be glitched in knockout mode. You’ll have to knockout the AI in two of the rounds to win the fight to earn your 20 XP. The glitch will stay active as long as your Xbox is on and you won’t have to repeat the steps to activate. Just keep grinding those quick fights. I’d recommend doing 50-100 fights in a session to speed up progress.

If you're anything like me, you've spent more than 400 hours having fun in Career Mode, Quick Fights and Custom Events, now reading this and wish you've spent more time playing Ultimate Team.
After A LOT of grind I finally got this done and can share some of my expirience with this time consuming achievement.

First of all and probably most important thing: Knockout Mode Glitch is the fastest way to grind this achievement:
Credits: 4PLAY Gaming

I ended it with 940 fights (898-42), which is 18,440 XP

My goal was to level up 5-10 times in one sitting. I needed around 8 wins. Mostly it took me around 45 minutes for 5 levels and 1,5 hour for 10 levels. That means it should take something around 22,5 hours to get this done.

I've had some kind of weird boost after getting to level 50 and 100. I was still getting 20XP per fight, but I needed only 2-3 fights to level up. After a couple of fights it came back to the way it was, but in 2 hours I was able to grind 25 levels.

I changed the weight class after leveling up just to keep things interesting. After completing a circuit of all weight classes, I changed fighters to try and have more fun doing this.

With the Knockout Mode Glitch most of the fights ended in 15-30 seconds of in-game time. I noticed that Woman's Bantamweight is probably the best for doing this, because most of the fights ended in 10-15 seconds. I just went crazy and started throwing combinations with Kowalkiewicz and 90% of the times fights ended very quickly.

I'm pretty good at this game and I managed to end my fights quickly, so depending on your skill it could take more time to level up.

I recommend keeping this as much fun as possible and remember that the longer you play it, the more mistakes you'll probably make. I really love UFC (I watch it every weekend) and I've had fun for most of the time doing this. Knocking people out was pretty fun, but last 20 levels was just painful and I couldn't wait to get this done.