as said above, expedition rifle of brut is your best bet. it has great bang for buck and is much better than mythic gun. I also have the magmatic totem and my experience with it is that its wonderful when the proc works but it happens pretty infrequently.

The rifle proc happens at almost every mob I encounter, sometimes multiple times causing AoE damage to everything nearby. With the totem, I usually see it proc maybe in 1 out of every 20 of my attacks. Its great when its works though as it panics a target randomly and then pulls nearby mobs into the target while damaging them (like Whim's arcane ability). And yes, it works on bosses.

However, In elites I still prefer to use the rifle because to get the totem to proc on a mob you usually need to be really close and thus you can get one-shotted easily. Therefore its not really practical to use the totem. Also l41 totems are at their cheapest around 300k for a potency one and can get higher than 2 mil for a brutality one. The l41 rifle of brut however can be gotten for 10k or less.