Not much can be said about this achievement except that it takes complete dedication, i devoted my weekend and some of Monday achieving this so it didn't take me as long as i thought it would but it really depends on your skill level.
Waves - In Robotron there are certain types of waves or levels that you have to beat in order to progress to the next, below i will describe the wave patterns and how to deal with them.
1. Grunt Waves - These waves are your bread and butter of Robotron, the grunts are red enemy that like to move diagonally and gradually get faster the longer you leave them alive.
A typical grunt wave will start with several grunts surrounding you and you must pick a spot immediately (don't change your mind) and start clearing the area of your chosen direction, if you die the stage becomes increasingly harder because the grunts get faster leaving you with less time to escape from the middle.
2. Hulk Waves - Hulks are the big green monsters found not only in hulk waves but many different ones too, they will block your shots (you can't kill them) but will stagger back slightly with every shot that connects)
Typical Hulk waves will start with lots of humans but with little chance to get to them as the hulks crush them on contact, the best way to survive a hulk wave is to kill the grunts and really concentrate on your movement making sure you don't walk straight into the hulks.
3. Brain Waves - Brain Waves are typically described as the hardest wave but it really depends on who you ask, a brain wave will consist of brains (that shoot bullets which dodge your bullets and are very unpredictable) and grunts.
From the beginning of the wave (and especially during later waves) it is advisable to go straight for the humans and collect as many as possible before you die, try to keep your movement sharp and unpredictable as this usually allows you to dodge the crazy brain bullets.
An interesting exploit which works only on wave 5 is as follows - Wave 5 starts with lots of mummy humans and only 1 child human (Mikey) at the start of the wave 9/10 the brains will all beeline towards Mikey and will ignore all other humans in the wave, use this opportunity to start killing any brain that is close to or going for mike whilst picking up every other human on the stage, if done correctly it is easy to amass over 100k before starting wave 6.
4. Tank Waves - This wave will start with lot of Hulks and tiny orbs called (Quasers) which if left alive will spawn tanks that shoot bullets that ricochet off the walls.
In this type of wave there are a lot of humans and inexperienced players will tend to run towards them rather than shooting the Quasers, this eventually leads to tons of deaths as every tank in the wave starts shooting at you.
From the very beginning it is advisable that you concentrate only on taking out as many Quasers as possible, less quasers means less tanks and less tanks means less deaths.
I know i have missed out a few waves and enemies but this guide was intended for the more experienced Robotron player (wave 10 or above) even if they don't find it useful i am sure that some newer players can use it as a stepping stone on their way to reaching wave 30 and beyond