General overview

Thorns is still the gold standard for consistent damage/AFK units, even years after his release. He lacks burst capability, but in return, he got consistent DPS that was unprecedented at the time. He remains relevant because of his 3x4 attack range, fast attack speed, and ability to attack drones. He can take a while to get set up on some maps, but once he gets going, you rarely have to worry about him afterwards.

E2? Mandatory, because skill 3 is his bread and butter skill. E2 also gives him the ability to heal himself.

Talents

Talent 1 (Nerve Corrosion) inflicts a poison effect on enemies Thorns attacks. Poisoned enemies receive a small amount of Arts damage per second for 3 seconds, and the damage is doubled on ranged enemies. This damage can really add up, especially against ranged units that you don't want coming near your frontline.

Talent 2 (Echoes of Ancestral Waves) restores Thorns' HP by a small percentage per second if he hasn't attacked in 2 seconds. This talent means you can leave Thorns to his own devices a lot of the time, though it's important to remember he can only heal himself when he's not attacking. If you need him to block strong enemies, he'll probably require healing support.

Skills

S2 (Protective Spikes) gives Thorns an ATK and DEF buff. He stops attacking but attacks up to 4 enemies in range (caster range) whenever he's attacked. This is an interesting offensive/defensive hybrid that increases his survivability dramatically while still letting him do good damage against mob enemies. The reflect damage doesn't count as normal attacks for the purpose of his talent, so he'll heal himself from any damage he takes.

S3 (Destreza) expands his range to a 3x4 rectangle, buffs his ATK/ASPD, and removes his ranged attack damage penalty. After the skill is activated for the second time, the multipliers are doubled and the duration becomes unlimited. This is Thorns' bread and butter skill. The damage is enormous, the range means you can put him basically anywhere, and he can kill many things before they become a problem. Another byproduct of the ASPD buff is that killing things faster means he has more time to heal himself with talent 2. You do have to babysit him a little until that second skill activation, but he's almost completely hands-off afterwards.

Masteries

S3M3 is a requirement for using Thorns to his full potential. I don't think most players need to do anything with S2, but the M3 gains are very good if you like to use it.

Modules

Module X gives Thorns an extra 10% Arts damage on his attacks. This is a nice benefit and really adds up with the multipliers on his S3. The upgrades add some more damage to his talent 1 poison effect, but, more importantly, they allow the damage to stack. This takes his poison damage from a nice bonus to serious pain if he attacks the same target for a while. I think mod3 is worth it because it gives him an extra poison stack.