To check your progress for this...
See how many you have under Goblins smashed.
All you can do to boost your number is choose a level with as many goblins as possible and knock down their towers or hit them with the shot.
2-3 has 7
3 in one tower, 2 in the second tower and 1 in the third, plus a goblin hanging from a balloon.
10-5 is very good with plenty to hit, but will take a while to unlock.
As a guideline I had the following:
Completing the game = 424 Goblins smashed
Gold on 20 levels = 467 Goblins smashed
Silver on 40 levels = 564 Goblins smashed
Silver on all levels = 745 Goblins smashed
Gold on 40 levels = 969 Goblins smashed
As always comments welcome.