I don't know if people are still having trouble with this, but I found a quick way to do this.
Pick either level 37 or 38, though 37 will cause less hassle.
Just continue through the level until you reach the third stage of growth or whatever so you can start eating sharks. Only eat the sharks and try to avoid the frenzy powerup. When you get near the end of the level (your bar is almost full) run into a bomb or clam (cannot be killed by a clam with a shield powerup). Continue this until you waste your 2 continues and then finish the level. You should net about 50 sharks each run.
Just an easy way I found, dunno if it's old or not. Probably not even a hard achievement, haha. Oh, and it took me about only 3 or 4 hours to get 1000 sharks with this method.