For those who want to get invincible (tho after that it's not fun anymore), just focus on the following, in sequence to each stage:
1. attack speed
2.burst+lightning
3.(if u r struggling to handle the fast orange snake) the one that gains health by attacking
4. (after 60)lava.
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Two bugs btw:1. when swipped/switched to a dif screen, leaving the game to run in the background, the fire stops attacking yet the enermy kept moving.
2. The lava remains to the next round if u r fast.
another suggestion is to allow 5x speed so the game is more interactive than passive