Diablo – do anything to improve the damage done by the demon summoned by the follower’s Maximus

Yes, it is indeed possible.

Take a look at this video. The Demon Hunter's Chakram clearly hits the Skeletal Archer twice, before obliterating it.

Look like I was mistaken. Upon further investigation of the Leoric (he has a fairly large hitbox) fight, it looks like the Chakram only hits once per target.

I had the same problem when I first faced these critters on my Monk. They are the first enemy that caused me to change my skill loadout.

There are Resist Poison items (easily searchable on the AH) that would help you against the insects in particular, and more armor always helps, but I found the right combination of tactics was all I needed to bring them down.

My best advice is to keep moving. The attack moves pretty slow and if you see it coming, it's not too hard to dodge. But as you said, all too often you are distracted by other foes. So here are a few tips and tricks to deal with those pesky flies as a monk.

• Dashing Strike is by far your best ability to deal with the flies. Don't be afraid to use this on the target with a line of projectiles coming straight at you. The dash will go straight through them and you won't take any damage.
• Cyclone Strike followed by a good AoE attack is usually enough to take them out. Don't use a knockback as a followup or you'll just have to start dodging and chasing again. I choose Breath of Heaven with the Circle of Scorn rune.
• Fleet Footed will help you dodge the attacks and catch up to the flies when they move away from you.

This probably goes without saying, but if you are playing multiplayer with a ranged partner, have them focus on the flies while you take out the other opponents.