"Now you can normally do it" doesn't sound like a helpful solution for people still playing this game.
This isn't as simple as advancing levels in the campaign and even beating the last boss from there requires strategy. In endless mode however, you not only will have to face the bosses again but also the regular enemies will get much more aggressive the more you progress, to the point of having some of them "magically" performing a mix of jump and teleportation up and down the screen and other crazy stuff.
Surpassing 999 meters will require you to advance about 5 levels in a row, but you can be set to repeat any of the 5 possible stages in the process. If you are unlucky, that means facing the two worse bosses twice: the icy rabbit and the boss from the volcano level.
Once you get the hang of it though, it's a very possible thing to do. Here's what I recommend.
Have a Gunslinger (fox faced ranged monster) fully evolved and with the following skills:
- Slow Motion (maxed out), Cooldown Mastery (maxed out) and Firework. This last one doesn't have to be maxed. The purpose of Firework isn't to solely cause damage, but also to buy time while the other skills reset. If you are smart when using your skills, specificaly Slow Motion and Firework, you can keep switching between them and that is key during moments of heavy adversity (hard boss fights or damaging large groups of enemies).
Brief and needed explanation about how the skill Fireworks works: it took me a while to figure this out since there was no ingame explanation for it. I tapped on the skill, there was an animation and then the character would launch a missile... which would go away without hitting no one, even if the enemy was right in front of me. No explosion, no fireworks. The thing is - and I accidentaly learned it latter - you have to shoot the missile to make it blow, but the best way to do that is to start shooting BEFORE he releases the projectile, after you tap to activate the skill. In fact, it's best to keep on jumping and shooting after activating the skill, that way the missile will go down when released and will blow against the enemies.
Other skills like Double Jump, Triple Jump, Atk Speed Mastery, Charge Attack and Resistance are all welcome too in order to improve your char.
The skills that I thing that the player should avoid spending points on are: Slam (for the Gunslinger, since he is a ranged char), Combo Mastery, Combo Extention (specially if you already unlocked the 999 combo challenge/achievement), Combo Frenzy and Bomb.
If you already have the equipment that I recomment for the Gotta complete 'em all achievement, that is, the Gold Pistol and the "Transformer" set (helm, back and boots), then you'll have the highest attack stat possible for this character.
As a pet, I had a lvl. 50 Magical Penguin with me when working on this achievement since his eventual healings are of great help.
With all of the above, now you can "normally do it". Things should get harder from level 3 onwards. Keep on jumping and shooting whenever there are flying enemies coming from the right side of the screen to stop them from shooting or setting traps on the ground. Use the Slow Motion-Firework trick to defeat the bosses. Use those skills whenever things get out of control with the regular enemies as well if you need a breather.
Each level will add up to the total distance. Only the first one will have 180 meters. After passing 999 meters you should get a notification and the achievement will probably pop once you are back to the main screen, like usually happens.
If this solution helps you in any way, leave a comment and a thumbs up to let me know. Thanks!