Arthur uses no mana and regenerates health every two seconds, so he can simply stay in lane. Losing a trade is recoverable by waiting for the regen, which means the game does not fall apart while you are still learning when to back off. His third skill is targeted — it cannot miss — and deals 16% of the target's maximum health, so it stays relevant against tanks.
Know going in — Ranged heroes can kite him down before he closes the gap, and with no shield or immunity, healing is his only answer to focus fire.