To hit E more frequently, throw E against the wave so that the explosion from the E roots them.

Q does more damage the closer you are. (Since Q damages each beam, you'll be able to hit more if you're closer to the enemy)

For more consistent W hits, aim it slightly ahead of the enemy's hitbox, in the direction that they will walk.

Right before you Zhonya's try to throw in an E so that they are trapped within your ult, and in general, try to use CD's before going into stasis.

To make the most out of your healing while in ult and in stasis, try to ult in a group with many enemies.

Go "Heal Swain" if you think you can quickly dive backline, and go "Fast Swain" if you cant.

PASSIVE
RAVENOUS FLOCK
Don't think too much about getting stacks as they should just come naturally through landing W's around the map, landing E's and killing enemies. Swain heals a considerable amount after his mini rework so use that to your advantage by healing in lane for sustain.

Q
DEATH'S HAND
Deaths hand shoots multiple beams that shoot out in a cone, so to get the highest DPS it's most efficient to attack closer enemies. (Think of your Q as a shotgun, it's not good for poke. Also, Q does a lot of damage so use it wisely as it also provides a lot of unexpected damage in lane as well as lane pressure.)

W
VISION OF EMPIRE
TLDR Swain W makes swain viable mid and should be used whenever possible so try to loop map a lot. This requires a bit of skill and game knowledge however since you should be using f keys to switch between your teammates. (For more information read Importance of W in the next chapter)

E
NEVERMOVE
Throw E at minion wave to hit enemies with the explosion that comes from the E as it is easier to land, and without it, it makes swains E very hard to land with its slow speed and small range (compared to other mages). E should also be spammed after the initial cast to pull enemies closer as soon as they are rooted, so not only can you do more damage to the enemy grabbed but also your teammates, note that you should only spam E if you are in a position to fight the enemy and if you are running away it's better yo wait before recasting, to get the heal while also spacing with the rooted enemy.

R
DEMONIC ASCENSION
A very useful fact that most people don't know about swains ult after the mini reword is that you can slow when recasting Swain's ult. Use this knowledge to your advantage by ulting when enemies are about to go out of your kill range (when you cant chase them). Swain's ult is also infinite as long as it's hitting an enemy champion so try to hover around enemies to keep your ult going as it heals you. (try to also stay around multiple targets to heal the most you can.)

Being probably the most useful ability in Swain's kit, you should look around your map as much as possible to look for W's to land as it is essentially a gank given to a lane while you sit mid. So to make the most out of Swain's kit look at the map as much as possible to land those game-changing W's.

Spam E as soon as you throw it so that they're closer for when you Q them to get the highest DPS possible.

After landing an E, while spamming E, W then Q then for maximum damage, as the Q will go off as close as possible and you're W which takes time to go off will have enough time to hit them.

Note that Swain's ult recast can slow, so use it if someone is about to exit your range and if you can't chase them. (Not having ghost/being slowed)

Laning Phase
Shove wave to hit level 2 first, then zone enemy off of CS and try to get as much damage in as possible while also looking for W plays around the map.

Mid Game
Give up mid to your bot lane then split push and W, anyone, you can find on the map. Farming JG is also a viable option as you need a lot of gold.

Late Game
Group like crazy and look for fights whenever, at this point, if you played the game decently you should be strong enough to just 1 v 9. (W side lanes to shove the wave in so that you get punished for grouping so much)

Fights
Run in and jump backline then after throwing all of your CD's go in stasis and after which recast your ult then continue fighting. (Play like a drain tank assassin and jump the enemy backline and kill everyone.)

Mindset
YOU ARE THE TEAM, YOU CAN 1 V 9, LET YOUR TEAM DIE, RUN IN AND KILL EVERYONE, AND IGNORE YOUR TEAM, THEY'RE ONLY HOLDING YOU BACK.

E W Q AA

Just kill them.