UFC Undisputed 2010

If you prefer to KO or TKO your opponents which I do then use Brock Lesner for this achievement. I tried the submission guides found above and a few scattered around on the x360a forums which I must say are very good but I could never get passed the 10th fight and would start to struggle around about the 8th due to the opponents reversing everything.
With Brock you should have hardly any trouble and will finish each fight with little damage. My 12th fight I went into it with 0% damage for each body area. Now what you need to do is make use of Brocks power so start off your fight with some big punches (LB + X or Y). I tend to spam LB + X up close as he throws some fast elbows. You should be able to get your opponent KO'd or at least gassed by doing this. Don't worry if they try and throw a few punches back because you'll be spamming X so fast the elbows will counter each time. You don't always knock them out straight away so the best thing to do next is to do a takedown (LT + RS towards opponent). I find doing a takedown better than a clinch and slam as when I do clinches the CPU always end up giving me knees to the face. Once on top work the transitions while throwing a few punches/elbows. This basically makes it easier to knock them out once you get back to your feet. Keep ground and pounding him and if you transition correctly you could get into open mount which you can do some serious damage from and 75% of the time KO your opponent. If at any point when on the ground the CPU reverses you then you must transition out asap and get back to your feet. I press LB + RS 1/4 turns from 12 to 9. If at this stage the opponent is still standing then launch a couple of bombs and when 1 hits it's game over for them. I finished all 12 fights this way in the 1st round with a total time of 22 minutes 14 seconds, all by the way of Knock out.

After trying over a dozen times with Shogun, Machida and Lesnar I was always flash KO'ed in fight 10-11 and never reached the final fight.

So I decidced to try a submission artist and breezed it my first try. My advice is choose Demian Maia (middleweight) and follow this basic strategy.

1. Run to opponent and clinch
2. Slam to the ground (which takes you to side control)
3. Land a few knees to weaken your opponent
4. Attempt a submission but do not work it (ie do not rotate right stick)
5. When the opponents stamina is in the red do a submission transition and work it as fast as you can.
6. Tap!

I managed to get Silva (Fight 12) to tap in less than 1 minute using this method and all the previous fights were almost as fast.

If this doesnt work first time simply stand up and repeat.

Just thought id add, not so much a solution because from comments to previous solutions its seems there is no straight forward fail safe way of getting this achievement. I tried the spam approach with brock and id get to about 9 maybe 10 before getting ko'd. My attempt at the submission reversal was pathetic i got to something like fight 5 and tapped out myself. So heres how i did get it.
I used Brock Lesner and basically took my time, all i used pretty much the entire time was LB-X Stay in striking range but also try stay away from the cage if CPU comes towards you, so just circle round your opponent. ALWAYS block your face. Work out if you CPU fighter tends to throw in single attacks or combos. For single attacks just block and wait for him to swing and miss, if he hits your block continue to wait cause he will more than likly swing again. A missed swing from a heavy attack gives you plenty of time to land a huge hook. Missed body kicks also opens the door to start your swing. Once you land just wait for the next chance to hit hard. CPU fighters that are more likly to combo i found there were less chances to hit home, just be aware that there might be more fists on the way so wait for the opening (missed kicks and missed heavy fists)
Another point which helped me a lot, take the time to learn your oppenent, the last thing you want to do is end up in the clinch with a good muay thai fighter, one of those knees land your asking for trouble. I found just avoiding the clinch all together helped the most, if it does happen just block your face and transistion for escape.
Ground game, if the fight is taken to the ground again just be aware of your opponents skills, North south, although looks a bit suspicous as to what they are actually doing in that posistion, it offers no submission to your opponent. On the other hand if you oppenent isnt very good on the ground, i had no problems moving to side control and mount, landing some knees and elbows from side can be very affective. Main tip on the ground is dont gas yourself cause if they do try submit you, you have no chance gassed. If you escape one submission just get up, dont stay on the ground cause you know they will try it again. I never attempted to submit them, it knackers your fighter for one and unless youve beaten them for two, three rounds they probably wont tap anyway. My last tip is one you dont really have any control over but if it happens then bonus. That being getting these guys in classes D and C
Frank Mir – 77
Cain Velasquez – 77
Shane Carwin – 76
Minotauro Nogueria – 76
Having to face these in later stages can be difficult, I got lucky and took out Mir Carwin and Nogueria before B then just having to face Mir first fight in A which was a little heart pounding. Especially when Kimbo Slice is fight 12, i figured if im gonna do it its this time round. Last fight i mounted Slice for a TKO win in the first round
My clear time was 26:26 for title defense mode. Using soley one move and escaping or aviodoing all ground and clinch fighting. Brock has a reach of 81 so my advice is use it :)

This is not a solution and its not guaranteed to get you your 50gs, above methods may work for you just fine, it just my advice, it worked for me when all other methods didn't.

i used lesnar ran out and started spamming lb+x for every match. finished in 12:16 with 2% damage. One guy gave me a problem but i just ducked his punch and overhanded him and went back to spamming.

When going for the career mode achievements, train your strength, speed & cardio to 70 then start auto training your attributes, get these to 50 in each then keep auto training & stockpiling them.

When you get near the end of career get your skills as high as possible & put all your excess points into defensive attributes & over 80 in punching offense (basically so that each stat is between 75-90). Along the way train up the long superman punches & some hooks. Now just go through title defense mode & keep clubbing the enemy & you should be able to power through this mode with pure cardio & punching power & defense.

I had a middleweight fighter at:

90 strength, speed & cardio, lvl 3 superman punch, lvl 3 left & ruight guarded hook, lvl 3 GSP head kick, lvl 2 napao head kick, 90 punch strength, 80 punch defense & the other stats between 75 & 82.

When going for the achievement I just kept spamming the superman punch & followed up with LB hooks & elbows, backing away when he was blocking & repeating. If he hits you with a combo back up & try some kicks, don't try to GnP the opponent in the last tier as they'll reverse you.

The reason I say to do the career fighter method is because I tried using the Demien Maia submission method & kept getting reversed on match 10 when going for takedowns or throws, the method I used allows good defensive stats & powerful strikes. All 12 matches were (T)ko's.

I cleared this with a time of 18:43 after five tries. Used GSP first but couldn't pass Fitch. Decided to try Lesnar and found huge success. The LB/X method worked for me but that wasn't the key. Eight matches ended as I spammed LB/X and when the opponent was dazed I hit LB/Y for Brock's very powerful uppercut. Got eight knockouts in a row this way. Only time I was dazed was the last fight with Velasquez. Reversed or blocked nearly everything I threw and that fight was pure luck. Did finish him using the same strategy, though. Thanks.

After trying all the guides above i could only make it to fight 10/11 before getting beat, then i tried with GSP and completed the achievement without suffering even 1% damage the whole 12 fights!!!

First run at your opponent and clinch, then slam to side control, from here you can afflict great damage by holding down LB and up on the LS and dropping massive knees to the ribs, rinse and repeat untill opponent is rocked on his back, from here you will automatically move over his head where u can land punches untill stoppage.

If the opponent begins to escape you will be in open top, from here click LSB and repeat the process again.

Helps that welterweight Tier A has below par challengers.

Hope this helps i managed to get the chieve in under 30mins, after countless hours trying the aforementioned methods!