Most waves have the normal assortment of Grunts, Hulks, Sphereoids and Enforcers. A good strategy is to kill off the Grunts in the immediate area, shoot the Sphereoids before they turn into Enforcers, then kill the remaining Grunts. The Enforcers are the biggest problem. They act much like Tanks and they are more deadly at long range. Both Sphereoids and Enforcers have a habit of sticking to the corner or edge of the screen where they are most dangerous. Stay away from the edges of the screen when they hide in the corner. Enforcer shots move along the walls when they hit the edge of the screen. Beginning at wave 28, a few Quarks are thrown in each wave (including the special waves) to add to the fun. Handle the Quarks just like the Sphereoids. Hit them before they change into the more dangerous enemy (Tanks). In all waves, your biggest source of points is picking up the Humans. Always go out of your way to pick up stray Humanoids.
Most beginner Robotron players will head for the sides as fast as possible. In general this is a bad idea, because you lose degrees of mobility necessary to dodge shots. Corners are the worst since you lose two degrees! One technique that is quite useful for dealing with quark discharge (a few of us Robotron players jokingly call it "candy") is to hang onto the wall just long enough so that the quark shots attach to the wall. Once they hit the wall, they will not leave it.
Every fifth wave is a Brain wave. The Brains are the most powerful Robotron enemy. They shoot deadly Cruise Missiles that follow you around. They also program your Humanoid friends into wild Progs that will kill you. Fortunately, the Brains only appear in Brain waves. It is difficult to complete a Brain wave without losing a life. Therefore, the best strategy is to use your first man to collect as many Humanoids as possible. Collecting seven will earn you an extra life. Be aggressive, go for the Humanoids even if it is doubtful you will survive. It's amazing what you can get away with if you try.
When the Humanoids are gone, circle around the outside of the screen. The Brains should be your first priority. They are usually in groups, sometimes right on top of each other. Don't waste time trying to shoot the missiles. They are so unpredictable that it is better to avoid them. Using two or three lives in a single Brain wave should still be considered a success. There are so many Humanoids walking around that you can easily make up for the loss of lives.
There are four different Brain waves: a Mommy wave, a Daddy wave, a Mikie wave, and a Family wave. The type of Humanoids in the Brain wave doesn't affect the strategy at all, but occasionally you can see strange things like Mikie riding piggy back on a Mommy, or twin Mikies holding hands.
On every brain wave there is a "target human" which all brains will attempt to capture. If you collect that human OR if the brains capture that human, they will immediately proceed to convert all other humans. One strategy to gaining lots of points is to kill off as many brains as you can and keeping that human on the screen until all brains have been defeated. On the first Mommy wave, wave 5, the target human 90% of the time will be the only Mikie on the screen. While the brains are appearing, search for that Mikie and kill off any brains near him immediately. Remember! Do not collect him! With practice, you can exceed over 100,000 points by wave 5 every time. This technique, of course, is harder on other brain waves, because the target human is indistinguishable from the others. You can train yourself though to detect which way the brains are headed!
Beginning at wave 7, every fifth wave is a Tank wave, consisting of only Tanks and Hulks. The Tanks start out as Quarks, which bounce diagonally around the screen. Quarks don't shoot, but they move unpredictably. Each splits into Tanks that do shoot at you. At the beginning of a wave, you have a lot of room to move around. Immediately go for the Humanoids before the Hulks have a chance to kill them off. Get rid of as many Quarks as possible so you don't get a huge army of Tanks later.
Once the Tanks start to appear, head for the center — the safest place to be is right in the middle of the action. The Tanks and their shots move slower when you're closer to them. You also avoid the deadly Tank shots that you have to dodge on the edges of the screen. Tanks shoot large balls at you that can be destroyed by your shots. To be safe, try to shoot them at the same time you're trying to avoid them.
A Tank wave is a good time to take a break without losing a lot of lives because each Tank has a limited number of shots. When you get to the last Tank, let it use all of its shots. Once it's empty, all you have to do is avoid the Hulks and the one Tank. Warning: if you die, the Tank gets a fresh supply of shots.
A nice approach to this wave is to try and stay in the central area of the screen. The reason for this is that tank shots (as well as quark shots) will come at you with much greater speed the further you are away from them. Thus, staying in the middle area will slow down everyone's shots giving you more time to dodge. You may be tempted to go for humans on later waves, but wave 7 is the only tank wave where a significant number of humans appear. Therefore, on subsequent tank waves (12, 17, etc.) it is advisable just to kill off the tanks as fast as possible and proceed to other waves where it is easier to collect humans.
Waves ending with 4 alternate between two different situations. One is a Tank wave with Sphereoids added. The other has mostly Hulks and some Grunts and Sphereoids. The Tank and Sphereoid wave should be played just like a regular Tank wave. Sphereoids and Quarks are very similar. They aren't very dangerous until they turn into Enforcers and Tanks. It's best to shoot them before they change, but try to get the Humanoids at the same time.
The other wave is a little different. The herd of Hulks roaming around makes it hard to move safely. You'll have to do some tricky maneuvering to avoid being trapped. Try to stay in the open areas and let the Grunts move into your line of fire. Remember, you can surround the Hulks and push them away with your shots. Most of the Humanoids are flattened immediately so this is not a very high-scoring wave.
Every wave ending in 9 is the infamous Grunt wave. More enemies are on the screen at one time than in any other wave. It is not as difficult as it seems, for the Grunts are the weakest enemy. They relentlessly chase you without shooting. Eventually they do get faster than you so you can't take all day to kill them. The key to surviving a Grunt wave is to make a quick decision right at the start. Pick one of the four corners and clear a path to it, then decide to go to the top or bottom. Never go straight to the right or left or the mass of Grunts will cut you off before you can get to the other side. When you get to the corner, follow the side wall to the next corner, then go along the top or bottom. You should be able to shoot enough Grunts while going along the side to allow you to make it across the longer top of bottom walls. Continue in a circle around the screen until you have the Grunts cut down to a reasonable number, then go after the Humanoids.
Deciding which corner to go to is the tough part. You have to do it as the Grunts are materializing. Your decision should be based on two things: the number of Grunts in the way, and the Hulks in the area. Occasionally you get an impossible set-up, but usually there is some way out of the middle of the screen. Sometimes the Grunts will appear to cheat and jump on you right away. The Grunts "jump" instead of walking smoothly so sometimes they move right on top of you before your first shot hits them. If necessary, don't move until your first shot kills the Grunt in your way.
If you get really cocky, try your luck at difficulty level 9. The Tanks are unbelievably fast even on wave 7. On a normal setting, once you can handle each of the different waves after wave 28, you are ready to play indefinitely. You are a true Robotron hero if you can survive all 255 waves (about 7,000,000 points). Wave 256 is wave 1 again so it's like starting over with whatever lives you have built up. At that point, it's just a matter of endurance.