This is A LOT easier than it sounds!
The aim of Classic Mode is destroy every brick in 99 levels in which it gets a tiny, tiny, bit harder from very simple to somewhere just south of medium as best. After this will come a guardian, unless you are trying to get the following achievement then you are unlikely to need to face this guy!
Now you are trying to get 1 million points, the easiest way to do it is to make sure that you destroy EVERY block on the screen using either the ball or your weapons. Do not use the Nuke unless you only have one block that you keep missing OR the ball gets stuck which is does quite often in the later levels. If you don't have a nuke then you can press the windows button and back out, when you continue playing you will be at the same level with the same points and lives as when you first started that level. (This can be done if you've lost loads of lives, but that honestly won't happen, as this really is very easy!)
Along the way to this achievement you will also pick up two other achievements.
You are unlikely to get any others as this part is so easy that losing lives is unlikely and you won't smash enough bricks, unless you have been playing other modes.
This achievement pinged for me in level 79, so needless to say I didn't play any further as that was for 100% completion.
**WARNING: Do this achievement EITHER before you start Revolution Mode OR once you have defeated 300 bosses and completed 100 bonus levels, as there is a glitch that can reset your counter on those achievements.**
has commented to say that for the first 30 levels your score will still be quite low, so persevere as you haven't done anything wrong.