Bullet Beat

Endless Hell mode is a step up from Endless Mode: endless enemies in the same wave fashion (again called levels), but there are a massive amount of enemies, as expected by the name. Fortunately, more enemies means more powerups, and since powerups make you incredibly powerful, they should constantly be your top priority. Don't even focus on enemies and where they are, or trying to shoot them. Focus entirely on scooping up all of the powerups around the center of the area and the enemies will die without even trying before your bullets will be shooting out in various directions. Hang out near the bottom and center of the screen, only venturing up or to the side to grab clusters of powerups. Staying in the center maximizing the impact of your bullets. You then simply need to survive long enough to reach level 10, which should be no problem since you'll be grabbing tons of health powerups along with everything else.

If you prefer a video reference for whatever reason, the video guide in Level 8-3 (50G) shows an abbreviated run of Endless Hell (after Campaign mode) to show what to expect and how easy it is to progress far enough for this achievement.

Additionally, there are achievements for reaching combo levels starting at 100 all the way up to 3500. This is the best place to get them all. It's virtually impossible to reach level 10 without getting a combo up to 3500, but if you do, keep at it until you get all of the combo achievements so you don't need to play through again.