I used second flag instead of speed, then sold loot/magnet after reaching 30k pts and replaced it with another flag, this maxed my dmg and fire rate, kept speed at 4 and this way I went till 50k pts at full lives, got bored and finally let 4th final boss kill me :P
Actually to get the BF badge you just need to move like 50 bubbles in one direction. There are multiple of these bosses so they aren't hard to be found further.