As a "mathy" person, Entropy is my favorite Sphere. Entropy, at its core, is the manipulation of probability. By increasing or decreasing the likelihood of an event occurring, the mage can twist reality into getting what they want. And because you're just changing chance, from the outside, you're just incredibly lucky, and so most effects will be coincidental. (To a point. Repeated uses strain credibility and could be considered vulgar.)
A simple example would be a card trick. Pick a card, then replace it in the deck and shuffle it a few times. What are the odds that the card you picked is now on the top of the deck? With Entropy, the mage can manipulate those odds so that it's the top card every single time.
When designing multi-sphere effects using Entropy, use the idea of manipulating the uncertainty factor in a certain direction. Examples:
Entropy/Forces: Reroute the flow of electricity in a building to overload the circuit path most likely to fail. (Entropy 1, Forces 2) (Can be coincidental if done in conjunction with the mage jiggering around in the circuit box.)
Entropy/Prime: Sense the weak spot in an object's Pattern -- the place most vulnerable to attack or exploitation. (Entropy 1, Prime 1)
Entropy/Life: The electrical impulses that generate a heartbeat are described as a chaotic system. By increasing or reducing the amount of chaos/entropy in that system, the mage can create arrhythmias leading to cardiac arrest and death. (Entropy 3, Life 3) (Had a Cultist assassin who loved this one. Depending on the health of the target, it can even be a coincidental effect: "Poor guy's heart finally gave out. It was just a matter of time.")