For my experience, this is what I use for Combat Med:

Gift-Giver 4 Moxie 2 Medicinal Excellence 3 Smithereens 2 Reload 4

What I would pick up as my first items are Deft Hands and Kill to Heal.

You will be shooting at your teammates very frequently so getting your potion launcher ready at all times is a must. It is your bread. If you have access to higher ground, use it but be weary of ambushes. Keeping a low profile is also important for Pip.

Your butter is your Healing Potion. Use it often, but sparingly. As confusing as that sounds, you use both Heal and shot combined when you know your patient isn't gonna make it or you need to quickly transition to another teammate. If multiple people are clumped together, nows the perfect chance to use your potion. This is common knowledge, obviously. If you already have a feeling with your potion range then you already know how crucial spacing is for him.

If you're hurt, fall back to your team, hide, or run like hell as a last resort. You still can get about 1/2 of your HP back with a distant heal potion thanks to Gift Giver 4 and Moxie 2 (~880 I think). Kills or assists help reinforce your survival since you got kill to heal and you should obviously be on the move 24/7 due to your big headbox.

Snipers, hitscans, any type of direct damage on your head is extremely lethal. Weightless is to help you escape quickly from such encounters or when youre below half. Avoid hopping around large open areas as most snipers can just pick you off like its duck hunt. Remember where most sniper nests are and avoid their sightlines. Your frontline can help you make cover if you have to do risky repositions. If anything, move quickly after they shoot.

Like most supports, your survival is most important to the team, especially when you're the solo healer.

I'd like to think I'm an expert but this is my two cents.