Mechanically speaking, Sonic Wave is a simple AoE magic nuke in the shape of a cone starting from your hero. The use of the spell is quite straightforward – it is your primary source of nuke damage and you need to use it to score kills and damage enemy heroes in fights.
It is important to note that the spell goes through magic immunity AND deals pure damage. This means that it is unaffected by magic resistance items and even more importantly – BKB.
It provides a slight knockback, so it could be used as a source of soft CC.
Although it has a big AoE, successfully hitting Sonic Wave can sometimes be a bit tricky:
There are two things to keep in mind about hitting your Sonic Wave – the range/radius and the casting animation.
First, the cast time of the spell is not instant (QoP’s cast time is 0.452 seconds), which means that Sonic Wave is sometimes hard to land, especially if the target is very mobile. You need to predict the movement of the hero. In case he is running in a straight line, you can simply click either on the hero or on the ground in a direction to the hero to hit him. If the hero is moving sideways you should predict where he is going to be in the next 0.452 seconds (your casting time) and use the spell in that direction.
Second, the radius of the spell is 100 at the very start of the spell animation and increases to 450. In case you are very close to your enemies, you might not hit everyone with Sonic Wave. Therefore, it might be better to step back and then use your ultimate to hit all heroes in the 450 radius. All in all, the cone shape of the ability and the big range (up to 1350) means that aiming it is far easier at the wider, further part of the cone. One way to abuse this is to cast your Sonic Wave first and to Blink-in to finish heroes with Scream, Shadow Strike, and attacks afterward.
It is also very important to note that Sonic Wave actually hits units up to 1350 range. This means that you don’t have to be within the 700 cast range to hit an enemy with Sonic Wave. Abuse this by targeting the ground when you want to hit enemies outside of your cast range.
When to use it:
You should almost always use Sonic Wave whenever you have the opportunity to score a kill. Your ultimate is your primary nuke, and it will usually be needed in order to get kills. What this means is that you should be ganking whenever your ult is off CD.
If you go for an Aghanim’s the CD of the spell will fall drastically (to 40 seconds). This means that you can start using your ultimate even to farm big waves or stacks (if you are fairly certain that you will not fight immediately after). Don’t over-do this, though. Not having your ult in a fight might lose it for you and at the same time spamming Scream is usually enough to take care of big creep waves anyway.