Alien Zombie Megadeath

Every time you set a high score after completing a level, that score will be added to a running total score across all levels that is displayed on the level select screen. Merely hit the required point amounts to earn these trophies (3M and 30M).

3 million is not very hard. You'll most likely have it by the time you unlock your 30th level. Survival levels bring the most points if you can last long enough.

30 million will take a little longer, but you should get it when you have around 110-130 medals. A good way to get a lot of points is to go back to the earlier Survival stages – you can get 2-5 million on them with a little practice. If you've unlocked 100 medals you should be good enough to get 30 million across all your unlocked levels.