The King of Fighters XIII

Go into Options and set:
* Rounds to 1
* Time to infinite (to avoid time bonus)
* Difficulty to VERY HARD

Picking hard-hitting characters like Terry Bogard and Goro Daimon and just spamming their jumping HK and crouching HK (sweep) worked beautifully for me. YOU CAN ACTUALLY LOSE when going for this achievement: simply pick the 25% lifebar option after the Continue screen, or whichever advantage you prefer to go into the next fight.

Pay absolutely no attention to the Target Actions. The fewer you do, the better. Without the added benefit of time bonus (which is the biggest contributor to your round score), you should easily be able to score less than 2,500,000 points. Should you get more than that, you will have to fight the bosses.

This is by no means a disaster, mind, since you can lose and set the CPU lifebar to 25%. It just drastically shortens the time spent getting this achievement and it might spare you a headache or two.

On Top of that, I suggest to use Goro Daimon and keep using 623+LP, setting the rounds in to 30 seconds. For those who feels panic for this achievement.

Not so hard.....

Play the arcade mode with the difficulty level set to 5. Just keep fighting. There's no penalty for continuing if you lose, so any time you do, simply continue and you'll be given 4 options that affect the next match. Select the option that reduces your opponent's vitality by 75% and go for the KO. Works on Saiki and Ash bosses as well.

A little lexicon beforehand:
B/DB/D/DF/F = back/down-back/down/down-forward/forward
U/D = up/down
P/K = punch/kick, L = light, H= heavy.
QCF/B = quarter-circle forward/back (down/down-forward/forward or down/down-back/back)
HCF/B = Half-circle forward/back (back/down-back/down/down-forward/forward or the inverse).
DP = Dragon Punch - forward/down/down-forward
(X)-Y = charge direction X for 2 seconds then push Y
close = your character sprite must be very close to the opponent (if not full contact).
EX : Extra move, use 2 punch or 2 kick buttons in the input. Costs a power gauge for a EX Special, and 2 for an EX DM.
DM: Desperation Move (called Super Special in the game, but I stick with the classic terminology out of habit). Special motion move that costs 1 power gauge (2 for an EX DM).
NEO: Neo Max Move, requires 3 power stocks and a full HyperDrive gauge (the green gauge used for cancels).
Cancel: entering a Super/DM motion during another move for a quick and powerful combo.
Chip damage: damage taken while guarding.
Guard roll: LP+LK while guarding, costs 1 power stock.
Guard break: HP+HK while guarding, costs 1 power stock.

ALSO NOTE BEFORE THE COMMENTS DEVOLVE INTO TECHNICAL DISCUSSION:
When I call a move 'useless', I mean by that if you are only an average player trying to get the achievement without building elaborate combos, I focus here on instant effectiveness, as such, subtle, intricate combos are not considered. Plus the CPU reads projectiles quite well on Very Hard.


Now, onto my solution:

I tried using Goro, but I didn't really get the hang of his mine punch (DP+LP), so I found another workaround that's much easier (at least to me).

It goes like this:
Unlock Saiki (get over 40 Target Actions by the end of stage 3, and beat him when he challenges you), then go to options, set the game to difficulty 5 and 30s round time.

Now go back into arcade mode, pick Ash, Saiki, and any character (I used Ralf because of his galactica phantom, his vulcan punch, and his diving air punch).

The third character will be your wildcard, because Ash and Saiki are beasts, especially Ash with his charge moves that will allow you to trap the computer, their EX moves are quite useful as well; Saiki, with his DP+K, has a very easy super cancel setup that works wonders (DP+K -> QCFx2+K). Their DMs are very easy to input, so it takes the burden of entering complex motions off your shoulders.

Your best moves with this team:

Ash:
Nivôse -(D)-U+K. Good anti-air, nice chip damage,
Germinal Caprice -(B)-F+K. Good for closing on an enemy, mind the recovery.
Pluviôse -QCFx2+K(K if you can connect the EX move, which does good chip damage, and can trap the CPU if it tries a guard break, but the punishment can be quite harsh if it doesn't let go of its guard)
Fructidor -close HCBx2+KK. NEO. Marginally useful, I could only use it once, but it helped me immensely, though it's definitely not mandatory.

Ventôse (B)-F+P is only useful to counter enemy projectiles and bait the CPU, very soon it'll block or roll past it. As such, Thermidor QCFx2+P(P) is a waste of time as well.

Don't even bother using Sans Culottes (LP-LK-HP-HK) and it's follow-up Germinal (QCB,HCF+PP) because you won't have time entering it, and you don't want to take any risks, sometimes, it's better to let the CPU finish you and transfer the power gauges to the next teammate).

Saiki:
Hanetsurube-no-Hata->Kasahazushi-no-Tsuchi -DP+HK->any attack. After a successful Dragon Kick, Saiki will teleport and hit the enemy again, usually pushing them further, giving you a breather.
Washiba Otoshi -QCFx2+K. Very useful and Damaging, also one of the easiest Super Cancels of the game, cancel Hanetsuturube-no-Nata into this by entering QCF+K quickly after the DP motion for a nice punishment.
Kyoryuu-no-Ori -close HCBx2+P(P). If you can close in on the opponent, it's a good move to input just after a roll, and it's quite damaging.

Shichiri-Gake (D,D+any attack) might be useful for evading or setting up Yami-Otoshi (Air QCFx2+K), but, like Hiori-no-Kusabi (QCB+K), it's definitely not mandatory for a good run against the CPU.

Tokoyami-no-Fune (QCFx2+P), Kasumi (QCB,HCF+PP) (NEO) and Kiyoku-no-Tsuki (QCF+P) are borderline useless, maybe just the 'projectile' spam can bait/chip the CPU into running up to you or counter their projectiles, but don't bother.
Kasumi being a NEO, the setup is not worth it, though it fills the screen nicely.

Ralf:
Vulcan Punch -P repeatedly. Very useful, good chip damage, your best friend if the CPU is cornered.
Dive Bomber Punch -Air QCF+P. Decent trap move, very useful for closing on an enemy.
Gatling Attack -(B)-F+P. Nice chip damage, OK recovery, EX does a nice combo.
Galactica Phantom -QCFx2+P. Excellent stopping power, I think it's still got some armor during the animation (I can bet the EX version does).
Bareback Vulcan Punch -QCB,HCF+K. even on the 6th stage the CPU can fall for this, though it's best used as a punishment or after a jump kick.
Jet.Vulcan Punch -HCBx2+PP. NEO. Marginally useful, if you have the setup, the chip damage is very good, but then again on a 30s timer why bother.

Also, classic stuff like jump-HP/HK into crouch HK works great (CPU can be aggresive and cunning on Very Hard difficulty, but it's nowhere near as absurd as it's been in past SNK games, in fact, even on Easy I find the CPU much cheaper and frustrating in KoF 12).
If you cornered your enemy, you can also spam some crouch LK before doing a HK.
Always keep moving, roll, block, don't just spam, the CPU is decent at learning if you use spam/turtle tactics.

Now, with such a small time frame you don't want to aim for KOs, try to aim for Time Up
wins, so as not to get any score bonus, and keep your combos low (save for the occasional EX/DM/NEO), try to rather use your power gauge for knockbacks (guard break) and guard rolls. If you can get the occasional KO it's all good, since it gives you more life back (DM/NEO KOs probably give even more, but with only 30s on the timer, building up a NEO can be tricky).

You can use (EX) DMs and NEOs, just try to perform as little Target Actions as you can so as not to get challenged at all, and keep your score under 2.5K, you'll get the bad end because you won't fight the bosses, but who cares, right?
If some stages are giving you a hard time, pick option 2 when you continue (CPU life down by 75%), but do note that:

1- It only applies to the first opponent in their team.
2- If they kick your ass, they'll gain life back.

All in all, it's not overly complicated, CPU fighting relatively fair for a max difficulty setting, just keep at it and keep cool, they have some big openings in their fighting patterns, it's really not that hard, except of course if you want to do a boss-including run.

Also, with the same team, using similar strategies, but on Very Easy, you can easily get


If you voted negatively, please let me know what I should improve.