A spell can mean a fairly or relatively short period of time; you'll hear weather forecasts predict a dry spell. Sports commentators will say that a sportsperson is going through a good spell when they're performing consistently better than they normally do.
If you have a crack at something, you try to do it. If someone is attempting to do something and they are unsuccessful, you might say, "Let me have a crack at it" suggesting that you might be successful at performing the task. ('Take a crack' is also used.)
A person may feel satisfied or pleased when they d something bad or unfair to you, but if you can get revenge, you will feel more satisfaction.('He who laughs last laughs best' is also used, and 'he' is sometimes omitted.)
To do something nice or kind to someone who has been nasty to you. If someone felt bad because they forgot to get you a Christmas gift, for you to buy them a specially nice gift is heaping coals on their head. ('Heap coals of fire' is also used.)