I have earned all 15 medals, so I'll offer you some tips.
General Info: Charge shot clusters as much as you can. Some players are beasts and can charge shot everything without locking on. It's okay if you can't do this but it will earn you a lot of points. Only use smart bombs when the enemy is spread out and a charge shot will not suffice. Destroy bosses as quickly as possible. Nothing beats a +10 hit and it usually decides the outcome if you're not a monster at this game.
Zoness: In the beginning, fire multiple charge shots at the water as fast as you can (there are a lot of critters hiding underwater in groups, so this will net you some points). If you can't manage this, use smart bombs (charge shots are still preferable). Aside from that just make sure you take out the searchlights and blow up everything on the boats. Sea mines won't earn you anything, so don't bother shooting them unless you're in danger of being hit.
BOSS STRATEGY: Nothing fancy here. Just make sure you take out his crane on the first go-around. If you don't, that's okay. Defeating him in under a minute shouldn't be too difficult. Remember that you have to destroy his smokestacks, cannons and crane in that order before you can completely kill him.
Titania: The weird giant quadrapod things will earn you +1 hits for every charge shot fired at them after they are taken out. Aim for the bottom and fire as fast as you can. Also, make sure you do not devote all of your time to the ground, 'cause there are a lot of baddies in the air too.
BOSS STRATEGY: SMART BOMB THE HELL OUT OF THIS GUY. They'll take out his limbs and reveal his weakpoint quicker than Fox can run in SSBB. Just make sure your aim is true because he can regenerate health.
Solar: LOTS AND LOTS OF BIRD CLUSTERS HERE. Take them out with charge shots! The rocks are fine but they're not always grouped so the birds should be your main target. Because you only need 100 hits, smart bombs are more acceptable here. Just make sure to blow up clusters with charge shots when you can.
BOSS STRATEGY: Very easy boss. Just make sure you have an upgraded laser. Killing him in under a minute shouldn't be a problem.
Aquas: This is the hardest medal to earn in my opinion (it took me forever). Do not attempt this level if you do not have an upgraded laser. There's not a whole lot of advice I can give you except take your time. This level goes very slow and hitting the breaks means you'll be able to get more hits. Some baddies like to hide and are tough to hit, so you might have to go through this level a few times to know what you're aiming for. Make sure you blow up clusters with torpedoes. Taking out the bigger enemies is vital because they'll earn you a lot of hits.
BOSS STRATEGY: This boss is a tad annoying because a whole lot of stuff is flying at you at once, not to mention your health could be dangerously low after the trench part. The stacks on his/her/its head can be taken out for extra hits. There are three in all, just don't lollygag.
EXPERT MODE: Same rules apply. You don't have to get more hits; in fact, I'd say it's easier to get medals in expert mode because there are more enemies to shoot. Just make sure you don't get hit too often because you'll lose your wings (and thus your upgraded laser).
Good luck mate. I'm not a genius but I've played this game long enough to know what works.
