|Unlike most GHB units, Garon has a ton of potential with his anti-anti-dragon breath. However, many dump it and replace it with lightning breath or water breath due to its seemingly useless secondary effect. With this build, both effects are utilized what I believe is their maximum potential. The only external factor that needs to be favorable is team support, especially a team that can provide Garon with defensive buffs. Given enough team support and perhaps even summoner support, Garon can be effective all of green, the majority of red, and even some blue units.
Garon's A slot of choice is Distant Counter. In dragon teams, it is vital in that it allows Garon to not only block, but take out mages with dragon-effective tomes. This will allow Garon's dragon allies to roam all the more safely throughout the map. It is also useful against standard green and red mages, considering Garon's decent Res stat.
Renewal 3 is Garon's B slot of choice. This may sound like an odd choice, but it has amazing synergy with both Breath of Blight's secondary effect and Garon's movement. With Renewal 3 and Breath of Blight, Garon will heal 3 turns in a row (not including the first and second turns). Coupled with his movement which allows him to run away to heal, unlike armored units, it provides Garon with amazing survivability.
Garon's C slot is technically flexible, but, for Garon's sake, should house some attack reducing skill like Atk Smoke 3 and Atk Ploy 3. I personally use Atk Ploy 3 since I gave Garon summoner support, but without summoner support, you may find that Atk Smoke 3 works best. The damage reduction, Garon's great mixed defenses, and ally defensive buffs all work together to ensure Garon's survivability.
Quick Riposte 3 is Garon's S slot of choice. This will allow Garon to take out most green units in a single turn.
Reciprocal Aid may be an ideal assist skill to use for Garon. With Garon's summoner support, Garon has high HP and high HP restore, making him easily double as a viable pseudo-healer. However, if that's not your thing, you can't go wrong with Reposition.
Lastly, Garon's special is flexible, just like his C slot. However, if you do intend to fill Garon's A slot with something other than a special accelerating skill, Garon should have a 3-turn special (on enemy phase, Garon will proc after being hit, retaliating, and being hit again). I personally find that Sol does pretty well at increasing Garon's survivability.
With all of the above, Garon is a mean-ass tank. Barely anything touches him, and in those cases, he'll just heal right up and pretend nothing happened. However, I cannot emphasize enough: he is HIGHLY reliant on ally defensive buffs in order to do his job well. So pair up your Garon with a Legendary Tiki (who has With Everyone) or another buffer and unleash chaos upon the FEH world. Sooner than you think, people, too, will be groaning in discomfort.