You should only really shoulder to finish a combo, tech chase, or some platform shenanigans. For an aggressive opponent Wario has no one true response that will cover everything, but he has a lot of different options. I'll list out the ways Wario can intercept someone, and some combos from the different options, depending on the weight and fall speed of the opponent, but really you're going to have to discover them for yourself. I won't list the things not specific to Wario, like shield grabbing.
1. Movement and aerial drift: Wario has the best aerial acceleration in he game. You really want to dash dance until they commit to an option and then jump out of the way, then drift back and punish. Granted you can't do this entirely off of reaction, you still have to predict what they are going to do.
2. D-tilt: This move is amazing for keeping someone honest. It's comes out and ends quick, as well as being a decent poke. Character depending, it can lead to more d-tilits, shfflc nair/ fair, f-smash, grab.
3. D-smash: Kind of tricky to land in neutral, but threatening with it's large range along the ground keeps people honest. Landing it leads to a tech chase situation, so it can lead to more d-smash, side-b, jab resets, or really whatever you like from a tech chase. Keep in mind if they are too close they can tech in place and be able to act before you recover form the end lag of the move
4. F-tilt: This is beautiful to bring out randomly around the third stock. It can be angled up to cover anti-air, and while it can't be combo'd out of it can out right kill at mid to high percents, situation depending.
5. Up tilt- An under appreciated gem, this move can protect you well when under a platform from an approaching opponent, and is a spacies magnet for some reason. Against fast fallers it can lead to upsmash, grab, up-air, or fart.
For general combo stuff, Wario has a lot of freedom for combos. Find what you like. Shfflc nair forever. Sour spot up-air into bite. Bite into DACUS. DACUS into up-air. Against floaties with little DI d-tilit into fart is fun.