Rogue Legacy

*MISSABLE*

The reason I have flagged this achievement as missable is because there is no free roam or any way to access your playthrough again after you beat the final boss. Once you defeat the final boss, you'll be forced to start New Game+ immediately, and you will have to meet the requirements below all over again to get this achievement.

This achievement is for beating the remixed version of Alexander. To access the remixed version of this boss, first you need to beat the standard version. Once done, you now need to find a Shrine Statue anywhere (on the current run or any subsequent run) and hope that the item you get from the Shrine is Alexander's Obol. This is complete RNG, both with finding a Shrine and getting the Obol, but my experience was that the game was pretty good about granting you the Obol when you were after it. Just ensure that once you get the Obol, you don't accidentally replace it with a different item from a different Shrine. Once you die and are back outside, talk to Charon to give him the Obol. This will open the boss door for Alexander in the Forest permanently (meaning on all subsequent runs, so you don't have to worry about beating it on this next run). Now, when you return to the boss door in the Forest and enter it, you'll be able to fight the remixed version, Alexander IV. Like all remixed boss fights, you play as a set character (so you don't need to wait until you level up more, and you can't build up your character to be stronger against the boss) and if you die, you're put right back outside the boss door with no penalty (you don't lose any money or progress, and can attempt again right away as many times as you want). 

Alexander IV is significantly harder than the original version, and in my opinion, this is the hardest boss in the game. All of the attacks are actually the exact same in this fight: Alexander still follows you and stops to shoot, and he still spawns Furies that also follow you and stop to shoot. The big difference is that the entire arena is surrounded with spikes, so there is no ground to walk on at all, just platforms to jump between, and on top of that, there is a giant spike wall in the middle separating the two sides. The only way through it is with  to dash through it, but you have to be close to the spikes before using it so you actually go through them. As a result, this fight is far more difficult to dodge projectiles and Furies. 

Thankfully, there is a relatively simple exploit that makes this fight far more manageable. How it works is, for both Alexander and his Furies, if they're far enough away from you, they'll just stay in place because they can't "see" you to come attack you. We can exploit this to separate Alexander from his Furies so you can fight by alone without any Furies bothering you. The other thing we're going to take advantage of is that he can only spawn a set amount of Furies, then he stops. So we're going to isolate him on one side of the screen so he can't see us, then lure all Furies to us to kill them (so the new ones spawn far away with him), then, once they're all dead, lure just him over while all his Furies stay on the other side of the screen. This allows us to just dodge him and his single attack while we focus all our attacks on him.

I will explain how exactly I was able to do this relatively easily and consistently, and then I will link a video showing it in action (but in my opinion, the video doesn't make it seem very simple and repeatable for actually isolating him). As soon as the fight starts, jump up to the top platform, walk all the way to its left side, and press  to place a shadow there to teleport to. Now hop down and go right, around Alexander, through the spike wall, all the way to the right side of any of the platforms (doesn't matter if you stay high or low). Kill off any Furies bothering while you wait for Alexander to come over. Once he gets to you, jump off the right side of the platforms into the spikes (don't worry, they don't do much damage to you). This should bring Alexander even farther right. Now stay on the spikes, and as soon as you see Alexander stop moving to do his projectile attack, immediately press  to teleport to your shadow on the top left side. As long as Alexander was on the right side of the right platforms, you should be good on the left for him to no longer come after you. You've successfully isolated him. 

Now the second phase. First, kill an Furies that may be after you on the left side. Now slowly work your way right on the platform, taking a few steps then retreating. This is to bait any alive Furies that are between you and Alexander to come attack you so you can kill them (so he spawns more farther off to the right). It is absolutely critical that you kill all the Furies before luring Alexander back over. So take it a little slow, and retreat each time a Fury comes so you don't accidentally go too far right. Once they're all dead (I've had to jump all the way to the spike wall and back to bait the last ones, at times), it's now time to bring Alexander over to you. Do the same thing you did with the Furies (I actually had to dash with  through the spike wall to get him to see me, before going back through to the left side) by going just far enough right to get his attention so he comes after you. As long as you killed all the Furies between you and him, he'll come to you and all the newly spawned Furies will be off the right side too far to see you.

With that done, it's now just you and him. Do an oval pattern to kite him in a circular fashion around the left side while hitting him, and avoiding his obvious projectile attack. It should be relatively easy to fight him like this and finish him all. Once he's dead and you open the chest, you'll get a medallion piece and this achievement. 

Here is a video of this isolation exploit in action. Full credit to the creator of the video.