This spell is pretty damn useful, and it pains me to see people refer to it as a trash spell solely because of its cast delay.
This isn't some poke spell, you're almost never gonna land it by taking potshots with it.
Use #1: **follow up damage**.
W has a 1.5 second delay, E has a 1.5 second root.
Land E, and landing W shouldn't be too hard.
300 (+70% AP) extra damage, plus the slow to keep you on top of them.
If you want to catch people further out, maybe grab a Twin Shadows to help you snipe people from afar.
You can also follow up on your allies CC.
Use #2: **path denial**
W has a radius of 265. If they're in the center of the spell, any unimpaired champion can walk out of this.
On the flip side, it has a diameter of 530. The only champions whose base movement speed is enough to get them out of this in time are Yi, Panth, mini-Gnar (with levels), and Cass (with levels).
This means that any other champion has to take measures to avoid the spell.
If they're out in the open, that won't do much for you, since they'll just step a little to the side, maybe closing the gap between you by 50 units.
However, if they're being forced down a narrow path, (the jungle or the gap between the tower and the wall, for instance) this forces them to either wait out the spell or tank the slow, which will allows you to catch up.
Use #3: **Farming**
A rank 1 W does about about the same damage to creeps as your basic attack.
Because of this, you can use it on the back line to prep them all for last hitting under tower.
Obviously, you can also just nab last hits with it if you're getting zoned.
Use #4: **Saves**
The range on this thing is what really comes into play here.
If an ally is getting chased, you plop this down between them and the threat.
They either have to change their pathing to dodge or tank the slow, both of which open a gap between them and your ally.
Use #5: **Trick them**
If an enemy sees you trying to land W on them out of the blue, they'll just dodge it.
After they decide what direction they're going to dodge in, throw an E to that location.
They don't have time to turn around and dodge the other way, but if they keep going the way they're going they're gonna tank your E-passive-Q combo.
Anivia does something similar by restricting enemy pathing with her wall to land her Q more easily.
Use #6: **Scouting**
Not exactly ground breaking, but still true.
Checking if they're doing baron, preventing your ADC from face-checking Gragas, keeping tabs on the jungler, it's all good.
Use #7: **Sustain**
OK this one is a bit niche.
A strat was floating around Reddit a few weeks ago.
Run spellbook, start smite, and buy a Hunters Talisman as your first item.
Whenever you get low, W raptors for a big heal.
Swain isn't the only one who can do this, but he can do it to either raptor camp thanks to his range.
On top of that, this allows you to get Runic Echoes if you want, which helps with Swain's movement speed issues in teamfights on top of helping with your mana issues.