Title Update 2 adds a new menu option--Endless Hell. The achievement is unlocked not when you reach level 2 in Endless Hell but according to some internal calculations. Moreover, the consensus appears that this achievement unlocks before the achievement for completing level 1 in Endless Hell. I unlocked this achievement when I reached level 7 immediately following unlocking the achievement for completing level 3. In fact, the achievements for completing levels 3, 4, and 5 appear to have the same time stamp. Three minutes later, at level 11, I unlocked the achievement for completing level 1. Note that different people are unlocking the achievements at different levels. Stay alive, keep playing, and all five Title Update 2 achievements will unlock probably around level 10 or shortly thereafter.
The key is to get lots of power ups quickly--especially shields. The game may start out slowly. That is deceiving. It will pick up quickly. Without power-ups, the enemies (that require multiple hits to destroy) will overwhelm you. Aim for clusters of enemies to generate clusters of power-ups. Aggressively, seek out the power-ups. If you are successful, you will soon become overpowered and will reach a self-sustaining state. At this point, the levels will advance rapidly. Since one hit spells game over, shields are invaluable. Keep your eye peeled for them and pick them up as soon as they appear. Once you reach the steady state, you can pick up shields continuously and can continue your offensive barrage.
The bottom line is there is a great deal of inconsistency as to when exactly the Title Update 2 achievements pop. It appears the Level 1 complete achievement is a typo and it should be level 10 complete. Levels 2, 3, 4, and 5 complete achievements might be accurately described. If so, there is a lag for many people as to when exactly they pop. If it is just a lag, it appears you must be alive for the achievement to unlock.