Found an Ultimate Guide for the achievement on TA so I thought I'd post it here to help.
Ultimate Battler 50
Complete Battle Mode
This unlocks upon successfully clearing all 20 Battle Mode challenges in "Battles" Mode. Battle Mode can be accessed from the main menu by going to Single Player > Battles. In Battles Mode there are 20 different kind of battle mode challenges with different conditions, some of which must be unlocked first. Most challenges require you to defeat a randomized lineup of 10 opponents and you must defeat all 10 of them with specific conditions for each fight such as: being poisoned the entire fight, playing as a random character, fight with only 25% health, mystery buffs, and more. Except for Classic Battle, every Battle Mode has its own set difficulty, and usually the difficulty level is connected to each opponent you face varying from Very Easy to Hard. Usually the first few opponents are very easy to defeat but as you move on the opponents will become more resilient and difficult to beat as you make your way to the final opponent.
*Updated 5/20* Injustice Patch v1.03 Release Notes
Battle Mode Breakdown
When first starting the game only 5 Battle Modes will be available, the other 15 must be unlocked. To unlock them you need to use acquired Access Cards and unlock them in the Archives located in under Bonus Features. You'll naturally get them when going for the "Hoarder" achievement. Access Cards are acquired whenever you level up and you should have enough cards to unlock the other 15 Battle Modes by the time you reach Level 20. At any rate, you should have plenty of cards while going for "The Hero We Deserve" achievement.
Clearing all 20 of these Battle Modes should not be too difficult at first. For 16 of the 20 Battles you can continue and select "Retry" if you are ever defeated. This allows you to have a rematch against the opponent who beat you so that you can continue where you last left off. You get as many tries as you need to complete the challenge. The only Modes you will have to clear without being defeated are Survivor, Speed Run, The Max, and Impossible.
You can see if you have already completed a Battle by highlighting the selection and pressing the Y-button to see the Leaderboards on that battle. Your GT will be displayed at the bottom of the leaderboard. If there is a 0 next to your name that means you have not cleared it yet. If you have some sort of score value next to your name that means you have cleared it already. Listed below are all 20 Battle Mode challenges and the conditions for each one:
Battle Mode Strategy
1. Classic – Defeat a series of randomly chosen heroes and villains set at your desired difficulty level.
2 Heroes Only – Can you stand against the might of the world’s most powerful heroes?
3. Villains Only – Test your skills against the most dangerous villains the universe has ever know!
4. Poisoned – Poison courses through your veins, constantly draining your health!
5. Survivor – Your Heath Meter carries over from match to match! Perform feats of exceptional skill to obtain bonus health.
6. Mirror Match – Fight as your opponent in a series of mirror matches!
7. Mystery – Each match grants a mystery buff or debuff to your fighter!
8. Fully Charged – Both you and your opponent have unlimited Super Meter but Super Moves are disabled!
9. Combo Heaven – You must perform a combo of the required number of hits and win the match! Transitions and Super Moves are Disabled.
10. Help From Above – Your opponents health will be restored to full every 30 seconds!
11. Injured – Begin each match with only one quarter of your maximum health!
12. Give And Take – You and your opponent gain health on each landed attack! You must defeat your opponent before time runs out!
13. Sidekick – Two opponents per match on a single arena will push your abilities to the max!
14. Speed Run – Can you defeat all of your opponents in under 2 minutes total?
15. Random Fighter – Fight each opponent with a randomly chosen hero or villain!
16. Countdown – You must defeat each of your opponents in under 30 seconds!
17. Unstoppable – Defeat all of your opponents without losing a single match!
18. The Max – You have one chance to defeat all of your opponents at maximum difficulty!
19. Full House – Try to defeat the entire cast of Injustice!
20. Impossible! – Attempt to defeat the entire cast of Injustice with a single Health Meter!
Using the character you're most comfortable with and hitting the CPU with your longest, most damaging BnB combos is a possible way to clear these Battles. For the #9 Combo Heaven
mission, be sure to use Aquaman
and use his Trident Rush move and the meter Burn version to clear it easily, it's done by pressing Down, Toward + X then RT. When doing this move press Toward your opponent to stay in range, you can move away or toward while performing Trident Rush to get more hits. Testyourmight.com has a number of listed combos you could follow if needed. However, if you struggle with completing any of these modes then a highly effective character to use for this would be Doomsday
. Simply jump at your opponent and press Down + A for a Body Splash to cross them up, then combo into this chain as you land: X, Y, A, Down, Toward + A then Meter Burn(RT) once the Venom Tackle connects for extra damage if you have it. This is literally the only move and combo you need in order to clear 18 of the 20 Battles and it can also be used for Survivor, Unstoppable, and Impossible! Feel free to make use of Doomsdays other weapons such as his unblockable Supernova, Iron Skin ability(B-button) and his Super Move if needed. Below in the video is the combo being executed as well as a successful run through Survivor Mode. Credit goes to Obie83 and dalleval for the combo upload and RA1NKING for the Survivor run.
In this Battle challenge you have to defeat all 10 opponents who are set to Very Hard difficulty without using a Retry. Many people find this to be the hardest Battle Mode challenge of the 20 to complete. The CPU has lightning fast reflexes and can literally read any move you throw at them and exploit an opening to combo you into oblivion. At times it feels like the CPU is perfect and knows exactly how to counter you and when. Most opponents such as Aquaman and Green Lantern can defeat you in just a few 20 hit combos. Luckily, there is a character you can use to make this more bearable.
By selecting Hawkgirl
you can simply spam her Mace Charge by pressing Away, Toward + Y to chip away at the CPU's health until they open themselves up for a Mace Charge for good damage. The move comes out quickly, inflicts good chip damage, and gets your Super meter up fast. Just be sure to properly zone some opponents by jumping back first before doing it and use your Super Move whenever you see the CPU is about to attack to gain the upper hand. Your Super Move is a definite X-Factor to your success. Play smart and stay out of corners because the CPU will pummel you dead. RA1NKING has posted a video of this method done so pay special attention to how he deals with certain opponents. Credit for the video upload and methods goes to RA1NKING, Dark Rocky X, PRSNightfire and themuffinboyy.
Using this move, you will still want to avoid certain opponents whenever attempting The Max. Because the 10 opponents you receive are always random, you can simply start up The Max, and if you get specific characters who give Hawk Girl problems, just press start scroll over to Battles, then exit. Repeatedly start up The Max until you get an easier lineup of opponents. Do your best to make sure you don't have to face any of these 7 characters:
2. Green Lantern
3. Lex Luthor
6. Wonder Woman
These characters can easily punish the Mace Charge on start-up with their special moves. They don't fall for the Mace Charge that easily and are too aggressive for this method to work. Although, they're not unbeatable, it will make your Hawk Girl runs go much smoother without having to face any of them. However, projectile users such as Ares, Green Arrow, Killer Frost, Sinestro, and Cyborg, you'll want to perform the Mace Charge up close. Once it connects, quickly dash toward them up close as they wake up then input Mace Charge again. Repeat, and the CPU should fall for it every time. An ideal line-up of characters you'll want to face in The Max using this method are
4. The Joker
6. Solomon Grundy
7. Black Adam
9. Green Arrow
10. The Flash
It's really all about match-ups and hitting enough Mace Charges and Super Moves for each fight-slightly luck based if the CPU doesn't behave properly. Be patient with this method, it does in fact work, but it will take some practice albeit maybe even a few losses to get it just right.