Jayce vs Garen Top Matchup
Jayce Top wins 45.5% of 1,597 Low Elo games against Garen Top, -0.2 points vs expected, 2-patch window 26.19-26.20.
Researched on LoL patch 26.20. Tips are written to hold at any rank; stats come from Low Elo ranked games up to patch 26.20.
How to play as Jayce
Keep landing ranged attacks to deny Garen's Perseverance(P), and save Thundering Blow(E) for when he uses Decisive Strike(Q) to close. Avoid a full-health all-in while even, since a complete fight can favor Garen.
Jayce's Mercury Cannon attacks can keep damaging Garen and prevent Perseverance(P) from restarting its health regeneration.
In Hammer Stance, Thundering Blow(E) knocks Garen back if he reaches melee range; Decisive Strike(Q) can remove slows but does not prevent this displacement.
How to play as Garen
Use Courage(W) to reduce Shock Blast(Q) damage when you cannot dodge it, and farm safely while Jayce has Thundering Blow(E) available. Commit to a chase after his knockback is spent rather than running into repeated poke.
Sources
These notes come from r/jaycemains and r/GarenMains, checked against the champions' abilities.