This should only take about half an hour (of rage) to accomplish the feat that is 100%ing El Ninja's beta. Patience is a virtue, my friend. This guide will be in chronological order of when to get each achievement.

Fresh Ninja
Reset your levels. Atleast you get something in return.


This is the first thing we'll do. Right after clicking the tiny start arrow on the title screen, you're prompted with a level select screen. There is a big red button that says "Reset Level" that you can press, even though you haven't even started the game yet.





Press the button and the achievement shall pop.

A-MAZE-ING
Complete level 11.


Go to the second page of the level select screen. Easy peasy.

Ninja'd
Complete level 1.


This one should also pop once going to the second page. But you'll need to understand how this game works to be able to progress to levels that are ideal for other achievements (i.e. GODLIKE!). The way this game works is that you launch yourself using the mouse. Here's a screenshot of how to beat the first level, it should help you understand how the game works a bit more.





The ninja stars are not technically required through your playthrough, in fact, I personally never actively went for them. They give you another move, which you have a varying limited number of in each level. If you run out, you lose. If you fall off the map or into the lava, you lose. To unlock Lover (the end of the level; functions exactly like Bandage Girl in Super Meat Boy) you must kill (in other words, touch) every ninja on the map.

Block'd
Kill 10 ninjas using blocks.


Basically, throughout the game there are a bunch of gray blocks that you can run into and make them fly into ninjas, killing them. It happens on accident all the time during a normal playthrough, and will VERY likely unlock throughout so. You need to just kill 10 ninjas with them. Here's another screenshot of what to ram into:
(from level 6)

Professional Ninja
Kill 50 ninjas.


This one doesn't even have an icon, which is slightly comedic for whatever reason.
Anyways, for this one you just need to kill 50 ninjas along your playthrough, which should be super easy to do and come automatically as I believe to get to the eleventh level you have to kill a total of 50 ninjas (not to mention, you're most definitely not going to finish each level first try).

GODLIKE!
Kill 8 ninjas in a row.


This one is a pain if you're actively going for it while playing each level. Fear not, you'll likely get it without trying on some of the levels with more ninjas within a small area. If you still don't have it, though, I recommend trying out level 6, 8, or 9 for this achievement (I did it on level 6).

Hopefully, you've managed to 100% the game! I hope this guide helped you get this easy hundo done.