Hello,
At the beginning of the game, you have to send zombies and, whenever you have enough xp, spiders. Then, keep spamming spiders until you have a good goldmine and enough xp for blazes. Later on, you can send waves of blazes. The final point of the game should be when you unlock magmas lvl 5, which are super efficient and can easily destroy the enemie tower.
You have to progress through all the game to, eventually, get better troops and spam them whenever the armageddon mode starts.
That's the strategie I use, and it's pretty efficient. If you want to know more about Tower Defence and troops, take a look at
this thread.