LVL 22, 46800
easy till then, once you get a strategy.
my strategy is to fire consecutively in the middle to make room for nexto ball + divide them + trap them
imitation of flOw.. no improvement over that. BTW, that round mostly-red ball isn't the boss. there's no boss but the strongest enemy is almost half the radius of the rooms. even with max level, it's almost impossible to beat this one because it has too many bubbles and the controls goes dumb.. (my laptop has 2Ghz dual-core CPU.) so I just keep getting out of the room. and when I return, he's new! if you're planning to beat that one, i recommend low quality option.
i tried assassin and gunslinger, but i recommend guardian. if you have high speed, in the midgame enemies will often MISS on you. but, later on that only rarely happens. you'll realize extreme strength is best. p.s. kill paladin many times, his stuff are great.