Your first XP point doubles your DPS for that Crusader. The rest are additive, so the multiplier doesn't add as much for each one you gain after that 1st one, but it can help you progress a little bit.
if it is too hard to beat to get impossible badge, here's a tip to make it easier (and read as a cheating play). in a handicaps menu, there are no death boom and pacifist mode and attack down series. in a cheats menu there are extra hearts and bullet speed down series. Checking these you will find 'Massive synergy'
To advice, I recommend you to upgrade allied unit in priority then upgrade your cannon and shields. If it reach Ether 100, now its time to improve your additional weapons; smart missiles->repulsion shield->secondary or triple shot. Upgrading Engine is the last process to explod yellow balls close to the red center
 "oO"){kind=icon}
 "Hero's Arms"){kind=icon}
 "GrindCraft"){kind=icon}
 "Crusaders of the Lost Idols"){kind=icon}
 "Cube Escape: Birthday"){kind=icon}
 "Bullet Heaven 2"){kind=icon}
 "Epic Battle Fantasy 4"){kind=icon}
 "Sugar, Sugar, the Christmas special"){kind=icon}
 "Doodle God 2"){kind=icon}
 "Idle Blacksmith"){kind=icon}
 "Hexagon"){kind=icon}
 "Feudalism"){kind=icon}
 "Haunt the House"){kind=icon}
 "Flight"){kind=icon}
 "Ether War"){kind=icon}
 "The Deepest Sleep"){kind=icon}
 "The Valley Rule"){kind=icon}
defeat the dragon at the end and then you can go back to previous places