SOULCALIBUR VI

I commented on the first solution but I figured I would write it down as a separate solution just so people can find it more easily because for it helped a great deal.

One thing you may want to try is yellow tea. It's a food item sold at the first town Dkaka. I don't know it if the town has to be be fully leveled first but it's there at max rank. What the food does is give you unlimited soul charge once it's activated. You still have to charge it up first by either combat or losing a match first but once it's activated you've got it for the remainder of the fight. I used that to spam the crap out of soul charge moves on the ancient on my second character and took them down a lot easier. The only downside is that it is sold for 7200 gold so it's costly but it helps a lot.

To get an Ancient to appear on the map as a Sidequest, you must max out the Weapon bar on the bottom right of the screen on Libra of Souls mode. Later high level missions will level it up faster. You will have it naturally by around Chapter 4 if you are using the same weapon.

Once the bar is maxed, a teal blue book icon appears on the large icon where Edgemaster's story events were, on the mid-right of the map. Go to it and do it to activate the Ancient sidequest.

The Ancient will be Level 70 and use the weapon you use. You must win 3 rounds against them to unlock the achievement.

The easiest Ancient to beat is Nightmare's weapon (Two Handed Sword). Have a Level 50-60 weapon and be about Level 50 or higher. Use Soul Charge and then mash forward Y for the sword "drill attack". Have 0.5 or 1 Second Soul Charge Gauge Recovery and Soul Charge gauge extension perks at minimum. Back off or hold down guard (A button) when you don't have Soul Charge to allow it to refill. Bring in food: 60% Health up, 25% Damage up, 0.5 Second SC Recovery, or 60% Health recovery on SC. A Mercenary can wittle down their health a little bit to give you an edge, but I never used them for help.

Beating the Ancient also unlocks Soul Edge weapon, which has 3 perk slots and what you should use to beat the 30 Astral Fissures and any remaining hard fights.

Please try the other solutions first, as this one is largely based on RNG and may take multiple tries before it works.

I found that I sucked too much at this game to beat the Ancient even with the Yellow Tea food item activated, so I'd share what finally worked for me.

I used Cervantes (the fighting style with one longer blade and one shorter blade); I was leveled to the high 60s and I had a level 50+ weapon. I also hired a level 50+ mercenary but I found that many times, the mercenary would die without hitting the ancient at all so it's not necessary to hire a mercenary for this solution.

The trick is to face the Ancient and hit back ( or ) depending on your orientation relative to the Ancient and hit + simultaneously. If done properly, your character should do a move where they teleport backwards, then unleash a spinning attack towards the ancient. If the attack connects, the Ancient is automatically knocked down; I found that I hit around ~200 damage each attack. I just kept spamming this attack on the Ancient to get him down to yellow health, then played normally to finish him. This strategy takes a while to learn the timing, but it's worthwhile because it also works for other tougher enemies, like Inferno or the Astral Fissure bosses.

Some tips to keep in mind:

Always try to keep some distance away from the Ancient when trying this, as the Ancient can break the animation/attack if they're too close; this attack has an extremely long range (basically the length of the entire stage), so just try to get as far away from the Ancient as possible each time he's knocked down. If you miss and he catches you in a combo, you can activate Soul Charge to stagger him in order to create some distance so you can continue spamming this attack.

Always keep track of your orientation towards the Ancient; whenever the attack connects, nine times out of ten you'll be facing away from the Ancient (with your back facing towards the Ancient); make sure you reorient yourself so that your character faces the Ancient before using this attack again, or else your character does some weird attack and you'll get caught in a combo.

Sometimes the Ancient will block the attack even if it's a direct hit; this attack will break his block so if he blocks, just spam and you'll still land a couple of hits and knock him back.

Sometimes if you're too far away, you can miss with this attack. To prevent this situation, I found it best to use it when there's around 1/2 the stage's length between your character and the Ancient and also when the Ancient is running towards you.

This solution is best done on an original Xbox One controller (with the 4-directional D-pad); I was playing with my Series X controller (with the 8-directional D-pad) and I wasn't able to hit the direction correctly each time.

The last tip would be to not only rely on this attack; sometimes the Ancient will get too close so make sure you parry/block instead of just trying to spam this attack at close range and get caught in a combo. You can also follow up this attack with normal attacks while the Ancient is down/getting up; this is pretty risky for the Ancient fight, and I'd only try it if a really good opportunity shows up (you land directly next to the Ancient after the spinning attack).

This method took me around 5 matches to get right, but keep at it and you'll beat the Ancient in no time!

EDIT: I found a video on Youtube showing this strategy:

(you won't deal as much damage as the person in the video as they're level 99, but the technique is the same)