(Spoilers) The Last Achievement?

[nextgener]

So I've gone as far as I can and have almost all achievement's except the last one. Anyone got any idea's as to what it is?

[Daraku Tenshi]

Beat the game on classic mode.
Then there is another achievement after that (which I assumed was not what you were looking for but...) Unlock All Achievements gives you an achievement.

[nextgener]

No, I beat the game on Classic, Boss Rush And 1-Shot mode.

[Daraku Tenshi]

Here is a link to an image that I made for you with all achievements maybe you are missing an earlier one like play the game without the gun: https://i.postimg.cc/vZGDk9ch/All-achievments.png

[nextgener]

That's The One! I need to finish the game without a Smart Gun!

Feel's like that one shouldn't be hidden. There's no way I'd have figured that's what you were supposed to do.

...also, how the heck do I that?

[Kaed✩Wolf]

You can get the Sword Wave upgrade before getting the Smart Gun or the blue magic.
Just get through the secret passage on Durridin Forest, get the hidden coin and jump to proceed to the right so you can enter in the cave (with blue magic blocks) from below.

[nextgener]

And Got It! Thanks for the Help!