Disclaimer I’m a silver Swain player but I do have mastery 7.
Early game: poke when you can but focus on farming mostly. Swain runs out of mana a little quick early on. Save e until they go to cs and trap em, aa, and q if there aren’t healthy minions in front. Once you get a hang of that throw in w. Best way to w is land snare, start w where the pull will go, pull them and walk up to auto and q. But remember if the poke combo is hard just keep farming. After 6 if your one enemy is too slippery look to shove and roam when ult is up. Swains w makes good for ganks. If they aren’t slippery let them push into you while poking them with autos when you can. Then pop that ult and use the combo to pull them in
Mid game look to stay with team. He’s very distruptive in a team fight and a lot of the time you can land w in clusters to get damage and shards. Hit whoever is close. Don’t risk jumping into back unless you have fully stacked shards and a bunch of damage.
Late game is the same. Team fight like crazy. People underestimate the burst you can do. Also don’t pull tanks if you can avoid it unless the enemy team can’t react as fast as yours. Try to save it to pull someone that is squishy.
Build: I go roa against burst champs or bruisers with mobility. If they don’t fall under these two I go ludens first. Second if I need defense I’ll go zhonyas and if they are heavy ap Banshees. If I’m not worried about defense second I’ll go liandry for tankier comps or morello for squishier. 3rd item ill Get the defensive item if I didn’t need it sooner or I’ll get the items I mentioned before. Then 4th if they are stacking mr I’ll get void but if not I’ll get deathcap. Honestly with items just look up builds.
Last tip, always fight with front line. Don’t sit too far back where you can’t land moves while your front line is doing their thing. Swain is pretty strong just learn ranges and kinda float within your team but still stay in range as much as you can.