- Note: This information is based on version 3.17 balance, and details might be changed by future balance patches.
The Apothecary is slow and fragile, and his role is first and foremost staying back out of harm's way and using his Healing Aura and the Heal ability to keep his Marines in the fight longer. Since the faction is designed to suffer few model losses to begin with, the Apothecary's base ability synergizes very well (some have said too well, in the past) with the Space Marine backbone units.
Heal is very effective on Tactical Squads, but the signature unit of the Apothecary player is usually the Assault Squad. ASM frequently get into situations where they can jump in but must get back out almost immediately, and Heal can completely turn these fights upside down. Instead of causing disruption and retreating, the ASM might now win the fight without falling back to heal up, holding the area instead and being back to full health in no time. Apothecary players will generally favor infantry units over vehicles, opting for a Librarian in tier 2 and Terminators in tier 3.
A skilled Apothecary should time his Heal to make maximal use of the effect, timing it just before taking a model loss. When Heal is on cooldown, the Healing Aura should be used to regenerate units in the meantime.
There are two popular Apothecary builds in the current state of the metagame, one ranged (clearly the dominant build) and one melee (more situational). The early days' builds with Advanced Healing and Tac spam have died out with new blobbing counters.
The ranged build consists of the Master-Crafted Bolter in T1, followed by either Combat Stims along with Improved Medical Equipment in T2. The MCB makes the Apothecary effective without risking himself and allows locking down enemy heroes with Full Auto, while the Combat Stims buff another unit (very useful on ASM, making them immune to War Shout or capable of chasing a unit in front of turrets). The extra energy from IME is kind of required in order to use all abilities frequently.
A variation of this build is picking the Armor of Purity instead of Combat Stims for simple Heal spamming, making the build easier to manage (one less ability to use) but having less utility. IME is still necessary to use Heal often.
The melee build consists of the Anointed Power Axe, Armor of Purity and Purification Rites. This Apothecary will counter melee units relying on energy – which the Axe will drain away – such as ASM, Bloodletters, or a hero using their "bubble" shield. Purification Rites will knock enemies away from the squad you are healing, letting them get free attacks while getting healed.
The Apothecary is the ultimate hero for reviving fallen teammates, and the only hero who can revive himself in 1v1 matches (excluding the Ravener Alpha's random resurrection wargear, which is, well, random) through the Larraman's Blessing global. However, the basic Heal ability will also revive an allied hero instantly from range. This method will not grant XP (and of course it wastes a Heal) so it should be used sparingly, but it can get an unconscious hero out from a very tough spot (such as an enemy base) without spending resources.