In order to get this ending, attack Moebius with your sword whenever he pops out of Morbvolt.

In order to achieve the good ending, during morbvolt's boss fight you have to make sure to only use your talismans on Moebius when he comes out of morbvolt you can still attack morbvolt as normal however. Do note that hitting Moebius in this phase at all will lock you out of the good ending unless you retry.

Boss rush is unlocked after beating the game. Overall it isn't much of a challenge on regular difficulty so not much strategy is needed. If you are struggling however make sure to make use of passive image pulses that give you buffs such as prevasion, more health, more damage, etc.

Getting an S rank in Gunvolt 3 really isn't nearly as difficult as it may seem. I would suggest loading yourself up with one or both "Reverie Mirror" Zondas to max your Kudos generation, paired with "The Muse" Lumen for survivability, and finally Mytyl to heavily reduce the Kudos lock imposed by using Lumen's image pulse. This strategy greatly helped me with getting high ranks even on more difficult stages. With your remaining image pulses just try to load out on other survivability options (ex. increased health). Make sure to also finish off bosses with your ex for that kudos bonus.

Refer back to the strategy I listed above. With some practice, it should be more than enough to get S+ on any stages you feel most comfortable with.

Now this is where you're going to want to really focus on your kudos game. This means trying to get as much air time as possible to max out as many combos as you possibly can with that 4x multiplier. I recommend the same setup stated earlier with some image pulses that increase air time, ex. "The Muse" Joule and "The Dancing Dead" B.B. As for stages, I'd recommend the tutorial stage due to a low bar to reach for S++ and Sunrise Palace 2 due to the many mid bosses you can get ex kill bonuses off of (try to channel your inner gv1 and get them both down to low enough health you can finish them both off at once with your ex skill)

What does Azure Striker rank entail you may ask? Well you have to beat an entire stage as Gunvolt, without ever switching to Kirin. This is done with the help of the "Primal Dragon" Gunvolt passive image pulse. Acquired after beating Morbvolt on normal difficulty, this image pulse allows you to begin a mission as GV, as well as allowing you to gain fetters gauge charge via kudos bonuses. Do keep in mind however that it also increases prevasion cost as well as forcing reloading state to only slow Fetters decay instead of freezing it. I would recommend picking either an incredibly short stage or one you're very confident in.

Hard mode brings revamped boss patterns and attacks as well as a minor health decrease among other small changes. Image pulses are still allowed so after understanding a boss better it should be more or less fairly doable. If you're really struggling with a boss, I would recommend saving Gunvolt to use exclusively on the boss and you should have little to no trouble. That is of course, until you reach Morbvolt. A common roadblock, especially with Octis Veto. You can, however, cheese Octis Veto by learning what stages he does it at and smashing him with your ex and/or skill image pulses before his damage reduction comes into effect (He gains significant damage reduction after the screen card appears). Do keep in mind also that On Hard mode and Very hard mode, you HAVE TO take the steps to get the good ending or else you'll be stuck fighting him forever.

Ah yes very hard mode. No passive image pulses, one use only skill image pulses and little health at your disposal. I would almost certainly recommend Kirin's XX trigger here as with the preexisting limits, it has no downsides. Gunvolt is also still usable, so try to pick your favourite/most comfortable stages and make use of these two factors. And Godspeed to you if you choose to do Morbvolt without cheese or XX trigger.

A classic. I hope you're ready for more image pulse grinding! Within the game, locked behind hardmode, there are various image pulses containing songs from past games to unlock, equip, and listen to, so you had better get on that hard mode grind if you hope to get these. You are not required to listen to the song in the joke ending to get this achievement, and if you really don't feel like listening to all of the image pulse songs, just having them counts towards the achievement.
Congrats you've done it! You've got every achievement in Gunvolt 3, a fairly impressive feat. Make sure to give yourself a good pat on the back.