How do I do this - Defeat 600 tier 3 or above units last shelter?

This task requires you to kill enemy players troops. (not zombies). You need to attack other bases and kill troops of the required tier or above. In this case you should look for bases that have at least tier 3 troops in them. You can scout them first to see what troops they have. Or assume that bases above level 5 should have some tier 3 troops.