lone rider is the best for score since you can farm minicoins against the flame punks and also lone rider gives more score in general
good lone rider route gives you around 600k+ score on ez while perfect merchant route gives you less than a decent stage 2

lone rider route start is probably the hardest stage start in the whole game but if you don't manage to get a chain up immediately it's fine and you can get it later against the wave of green planes after the first punks
just make sure you get it soon or your score takes a huge hit

don't shoot bullets against the green planes, basically ever (especially if you have a multiplier)
they are super fragile but also don't really move so they're good for shooting rockets at
optimal time to spam rockets is during the lone rider preview because there are a ton of green planes

against the big orange planes, fire a big rocket at them to kill them while making them not super annoying
if they fire rockets, use bullets to clear the way for your rockets to hit the green planes

against lone rider, don't kill the first mounted turret in phase 2 immediately so you can farm minicoins
in final phase, you can damage his wheels while he's running backwards to equip the flamethrower
remember that you are immune to bullets and fire while inside your own rocket explosions so you can exploit this to sit inside the flamethrower to farm minicoins and damage his wheels
killing bosses results in more score than sparing them i think so make sure to kill lone rider (and destroy the flamethrower if possible)

lone rider route is fairly volatile though and it's a little easy to screw up, especially if you're attempting to go for a multiplier right at the very start instead of waiting for a little bit

don't get discouraged though and practice a lot
previous stages don't really affect this stage if you're going for score so practice mode is good