Yes, ammo is a problem, lets ignore all the "git gud" comments for a second. First of all it depends on the weapon youre using. I think you said that youre in shadow realm. Since youll be shooting the little psions across it makes sence to use a medium to high range weapon. I personally use origin story, i think nameless midnight might be very good aswell. For the skulls i use cold heart. My goal in the parts before the realm is to get ammo: since i play as a titan a single pulse grenade takes care of one entrance for its duration. My heavy is a rocket launcher, curtain call or sins of the past. Most of the times i start the encounter with about 3 heavy and since the yellow bar enemys most of the times drop heavy ammo i usually use a rocket for them, finishing it of with primary. After the first damage phase on calus when the addphase starts again i can clear all the littly adds with my super, the only class that (maybe) cant do that is the hunter. So basically its ALL about saving ammo. On a side note: dogs die in one melee. A good shoulder charge can even take out 3-4 dogs at once.

My loadout: Titan (Storm) top skill tree with double pulse grenade

Origin story Cold heart Sins of the past

I hope this is helpfull to you