I know this is quite a while ago, but I have at least found the path to getting to wave 50. I think I can say that I have good knowledge of this game now.
Weapons:
-- Assault Rifle: balanced weapon, but it tends to run out of ammo quickly.
-- Shotgun: good damage at a short distance and can destroy multiple targets per shot. Lots of ammo. However it's not very good to use against most enemies. Useable against assault troopers, minelayers, missile boss, and melee boss.
-- Sniper Rifle: very long ranged weapon, but it has low ammo. Can pierce and destroy multiple enemies. When switching to this weapon, your character will reset his aim towards the bottom of the screen for some reason, making the weapon swap DPS trick unusable. While using this weapon, your vision will be extended, just as if you were at the Atlantis.
-- Sword: eh... personally I find this to be very difficult to use, and it runs out of ammo quickly. However, you can zoom across the battlefield ridiculously fast!
-- Missiles: homing, with good damage. However, its speed makes it ineffective against melee troopers and minelayers.
-- Vulcan: basically a mini assault rifle with a shorter range and weaker damage. Its particle shots can be pretty difficult to see.
-- Atlantis' Cannon: the ultimate weapon! It can only fire underneath the Atlantis unfortunately (indicated by dotted white lines). When selected, click once to aim and fire. It will continue to fire until you either deselect it or it runs out of energy. If it does not fire for a while, it will regenerate its energy. Ideal for taking out bosses quickly, but it can also be used to destroy huge enemy swarms and snipe siege turrets.
Enemies:
-- Assault trooper: basic enemy with an assault rifle.
-- Melee trooper: circles around you, usually out of reach of the shotgun, and will close in occasionally to attack.
-- Minelayer: rushes at you really fast and leaves behind green mines. The mines can be destroyed.
-- Siege turrets: they only spawn on the underside of the Atlantis and fire red bullets from really, really far away. It does not take much to kill them. Chances are the very first time you encounter them, you'll see a shower of bullets "floating" towards the Atlantis.
-- Sniper trooper: uses a sniper rifle, which can hit for quite a bit of damage if you're not careful. They tend to shoot at you first if they spawn within sniping range. Otherwise, they'll zoom into a location close to you and then try to shoot you.
-- Advanced trooper: similar to an assault trooper, but they have more health, they spray red bullets, and they explode upon destruction. This explosion is fatal if you're too close.
Bosses
When a boss is active in the area, the music changes. All bosses spawn within range of the Atlantis's cannon, although very occasionally one will spawn just out of reach at the side. That sucks. Also, near the end of the game, you don't have enough ammo to kill one, making it essential to manage the Atlantis's cannon.
-- Carrier: has a crazy laser spray and a turret gun. If you aren't super beefy, you're probably going to die for even getting close to one. Fortunately, you can kite it with an assault rifle. Once destroyed, it will release a couple of enemies.
-- Melee boss: basically a super melee trooper. It doesn't like holding still, so you'll want to lead it by the nose when aiming the cannon. If it attacks you, you might die. Quickly escape using the sword.
-- Missile boss: it shoots missiles similar to your own. Missiles can be shot down. Otherwise, it serves as a damage tank and is fairly ignorable.
Game dynamics and milestones
Early waves:
-- Get an upgrade level in the repair bay tech. This will make repairs and restocking twice as fast!
-- Focus on upgrading your weapon of choice first. You'll want to ramp up your DPS using the cheap and quick way. Unfortunately, this maxes out fairly quick, but at least it'll carry you through until you have the income to seriously crank up the Atlantis's firepower. I would recommend the assault rifle, but maybe you can have just as much success with the other weapons? Who knows.
-- Bosses will spawn sporadically, and you may encounter up to roughly 5 bosses in a single wave. Make sure to take them out, but be sure to help with the minions if they're overwhelming your defenses.
Mid waves:
-- You should be diligently upgrading your investment techs: multiplier and upgrade bay. Multiplier increases the amount of points you can spend from enemies, and the upgrade bay makes upgrades cheaper. Complete these two upgrades by wave 30.
-- While they are not your top priority, make sure you invest points in defenses and the Atlantis. About 4 or 5 points into turrets will allow them to hit most things reliably and deal good damage.
-- Wave 30 and beyond sees a large spike in enemy strength. While they are not necessarily more numerous, they will have a significant increase in health, power, and speed. Wave 33 however has a huge spike in enemies, and will prove formidable to overcome.
Late waves:
-- Bosses cannot be destroyed by your own weaponry alone, at least not in one trip. Get good at aiming the Atlantis's cannon weapon, and make sure to manage the siege turrets if they spawn.
-- Regeneration and the Atlantis are the techs you'll want to focus on. With a high regeneration stat, this significantly increases longevity of AI Mermen, as well as give the Atlantis a serious damage buffer. Everything should be fully upgraded by roughly wave 40 or so.
-- At this point, your own weapons will be fairly insignificant. However, the Atlantis should prove to be such a formidable force, that it can defend itself from any amount of small enemy attacks. But don't rest just yet! Your primary job at this point is to eliminate siege turrets and kill bosses.
