TNA iMPACT!

Right after I beat the game and had all the TNA characters unlocked, including Senshi, I went after this achievement. Let me first say, it DOES WORK. And I'll also say that it can be very easy to get, depending if you want to make it easy or challenging for yourself.

This was my strategy for getting it:
1) Before attempting to get this achievement, make sure you have completely cleared the Story Mode first, so you will have unlocked everybody up to and including Jeff Jarrett. Additionally, be sure you have also purchased Senshi, who you can unlock at 200,000 Style Points.
*You DO NOT need to beat Created or DLC characters.*

2) Plug in a second controller so you won't have to waste time actually fighting each CPU opponent. (If you plan to actually fight the CPU, disregard step 3 as it will not apply to the CPU opponent.)

3) Select Shark Boy with the first controller, and then select an opponent with the second controller (I found it easier to start with AJ Styles, and then just go down the entire list). When the match starts, do Shark Boy's running grapple, which is the Hurracarana, and then quickly pin the opponent. Since the opponent is being controlled by the second controller, you will score an automatic pin.
*Tip from Motley Gunner*: "you should add that doing this at the mexico arena cuts out ring intros and their loading screens which saves about 20 secs a match (26 matches X 20 sec saves 8.6 minutes, thats a lot of loading)."

4) Repeat Step 3 for the rest of the roster, until you get the achievement. **REMEMBER, this must be done in one sitting, so DO NOT turn off your console until after getting this achievement.
(Credit goes to DaOutsider for stating this in his solution below.)**

*NOTE* If you have not previously gotten the Mid-Card Contender Achievement (for winning 25 matches in any mode in one sitting), it will pop for you at some point near the end of the roster.

*If you have any questions, concerns, or comments about my solution, feel free to comment here or private message me. Thanks.

To unlock this achievement YOU MUST DO IT IN ONE SITTING. If you turn off your Xbox halfway through you'll have to start all over again.

The only wrestlers you DON'T need to beat are:
Benny
Lenny
Afro Thunder
Don West
Mike Tenay
+ Any created wrestlers

I beat all the other wrestlers in a one-on-one match by pin.

frankyrulz is WRONG.

I got the achievement last night.

Simply download the live patch and for good measure the free Tenay DLC on marketplace.

Then simply play against all the wrestlers including Suicide.

The ONLY unlock wrestler you must face is Shenshi (Low-Ki) at 200,000 points.

Done.

Credit goes to YouStayClassy @ x360a who clarified it on the forums and helped me get mine.

You just need to beat the following wrestlers with Shark Boy

Abyss
AJ Styles
Alex Shelley
Booker T
Brother Devon
Brother Ray
Chris Sabin
Christian Cage
Christopher Daniels
Eric Young
Hernandez
Homicide
James Storm
Jay Lethal
Jeff Jarrett
Kevin Nash
Kurt Angle
Rhino
Robert Roode
Samoa Joe
Scott Steiner
Senshi (unlocked at 200,000 points)
Sonjay Dutt
Sting
Suicide
Tomko