Bloodstained: Curse of the Moon

To accompany the other solution on how to avoid sacrificing characters, this achievement can be completed in the Normal game (playing solely as Zangetsu from start to finish) and on Ultimate Mode. Ultimate Mode gives you Zangetsu with all his abilities plus a powerful Charge Slash (Hold until you hear a chime and you'll get a stronger attack). To unlock Ultimate Mode you must complete the Normal game as Zangetsu having sacrificed each character.

The upside to doing the Ultimate Mode method is you won't have to be constrained to Zangetsu without use to his crescent slash and double jump. Both of these skills are highly useful, without them you'll be running base Zangetsu from start to finish. Doable but probably its own set of difficulties in contrast to Ultimate Mode.

The downside to this mode is that the bosses are stronger and have their Nightmare Mode behavior included. This also includes a unseen version of fighting the final boss of the Normal Game (the demon bat girl with moon powers), which is a real pain due to the speed at which her attacks move. Fortunately they're all very much beatable with just Ultimate Zangetsu.

As I have no idea what any of the enemies are called...I'll just go by "Stage #" and something I identify them with. Note that I tried at all points to use a Charge Attack, as you can hold it and move around including jump while waiting to throw the attack either from a standing or airborne position. Secondary Note - do not worry about final blows, in my experience I was down to one health after getting the "chime" that they'd been defeated, then I was hit multiple times not dying.

Stage 1 Robot on a Train

This enemy has his same basic attacks with a little bit of extra flare provided. By example, when he fires the multiple fireballs into the air they will now leave small pits of fire on the floor (that you can clear out with an attack) and lead to possible damage if you step on them. At the end of this attack he generally fires a large blast from his chest. Other than that, just attack his face a lot either standing on the steps he pulls to him or abusing Crescent Slashes/Charge Slashes while double jumping.

For the final blow, hide behind the steps he pulls in to shield from the massive attack.

Stage 2 "Turtle"...?

More ice cycles hang from the ceiling and the dashing attack shoots up higher shards (double jump required to dodge). When the shards fall from the ceiling, go a block left of the rock that's falling to avoid damage and jump to either direction when the "turtle" lands. There are what seem to be more bubbles that moves faster. Avoiding damage can be done by using charge slashes and taking out as many bubbles as you can.

For the final blow, apparently you're supposed to hit her as she charges but I just tanked it...so...important for Boss Rush?

Stage 3 Hell's Monopoly Man

This fella doesn't have much in the range of attacks, as always jump on the gold coins and attack his head. When the hands come together for a Donkey Kong styled smash, just avoid contact and get some hits from the gold that shoots up. Another one of those rock monster things appears too but just avoid as you normally would. Not much more to it.

His final blow is the easiest to dodge just move in the empty area of raining Gold until the bright light flashes.

Stage 4 Purple Dragons (not Spyro)

These fake Spyros both shoot firebeams and fairly frequently when you're on their level. Bottom one is more active on biting but everything is pretty much the same. Take dragons out and the third arrives. For this one you'll need to jump down more often to the level above the spikes and DUCK, as the flame shoots lower this time. Be quick about jumping back onto the other platform as the spikey tail joints move a bit quicker.

Final blow uh....wait, what final blow? Did they forget one for these things? It just explodes that's it.

Stage 5 Electric Bird Knight? Thor Cosplayer among bird people? Is it even a bird...? The thing ontop of the mast of the ship.

Right this guy...he's not really any different. I guess the beams are a little more deadly on his dash attack but...I guess he just has higher health like all the other bosses.

Final Blow...well it's avoiding his final dodge then standing a little ways from either the left or right edge so you'll be between the light electric beams and the big death one in the middle.

Stage 6 Bloody Mary Poppins

That ball thing is a bit more active, just make sure to strike towards it to be safe. I would always go to the top two umbrellas (hit them to make them stay still) and attack her as she moved around. Just avoid being on the floor and jump down to the second level umbrella when she gets close. During her bloody rain attack, go beneath any of the Umbrellas (preferred a stunned one).

Her final blow I don't even know man, just tank it. That bloody vortex takes up the whole place.

Stage 7 Mecha Gex

Move up to him and slash a few times then jump to avoid the bordering green electric field he creates. Keep slashing him on the platform here then pursue to the next. When he starts targeting platforms as always move to the one he hasn't and if any green spikes approach break em. After he does his rapid around the room attack, jump quickly to one direction and land to avoid damage from his attacks.

His final blow stand ontop of the center platform then double jump up to either corner more towards the center and you'll avoid the four Gecko attack.

Stage 8 Giant Succubus

This thing...is a pain. The first stage you'll want to just slash, I didn't even bother with Charge ups and hang more towards the left side. It just takes luck to not get caught in the purple wave thing that crosses the screen from time to time. Try not to stand too close to her as her bubbles are greater in number and move way faster. If it's green you can generally stay inside of her attack and slash. I'll be honest, only a handful of times did I manage to beat this part of the fight with at least half health. I tried to tank certain areas and just attack as much as possible.

Second battle....alright so her sickles work differently now. They're faster and the pink ones are a bit harder to catch (but possible) with a crescent slash. Attack the best you can when she uses these...the yellow ones, I found that I finally used the dodge attack. Move to one side of the tower and time it to dash to the other side so both sets move there. Then quickly crescent slash them all and prepare to dodge to the other side to avoid the next set of sickles. Destroy them as well. Her big attack where she throws a friggin moon at you stay towards the center and jump once to avoid the energy on the floor then move to the left or right to avoid the moon hitting and at either corner while the attack finishes.

Final blow...well...you deliver it, stand in the center and let the vortex of flames engulf you until you pull off a pretty sick vertical cut.

A final note is to play on Casual, you're just in this for the achievement anyways so don't punish yourself with knockback and limited lives lol remember playing as just Zangetsu means one life. So either do the Normal game with Zangetsu's standard attacks or do the Ultimate Game to have all his moves and fight tougher bosses. They all seem to have more health so don't be alarmed when it takes multiple Charge Attacks to kill any of them.

In the first 3 levels of the game, after you beat a boss you will encounter a captive character/ally/adventurer. Just walk past them - DO NOT PRESS NEAR THEM NO MATTER WHAT - to the far right to move on to the next stage. "Sacrificing" a character means you attack and kill that character instead of recruiting them. Again just walk past them; do NOT attack or recruit them.

From there, play through the game to the final boss and win.

Good hunting.

TVpunishers solution is spot on forultimate mode but final boss on ultimate is brutal. So as a caveat to the topmost solution:

I kept failing on final boss on Ultimate mode so I did a run through on Normal mode and it was FAR easier. Even without the charge slash and double jump, having the bosses have like half the health of Ultimate mode and do far less damage made it way easier, not to mention their attacks are easier to dodge. Beat every boss 1st try on normal mode whereas Ultimate took several tries for many of them and final boss kept wrecking me.

For anyone struggling on this one try doing it on Normal mode.