two shot Callahan vs Dodgy beasts and Fiends?

Ok, i now have decent taste of what the magnus lines can do (got mega tundrus and mega magnus at lvl 10). Devastating is the only word that comes to mind. I'm ditching my antigua guns for these babies.
With that in mind I'm planning to upgrade my mega magnus to Callahan to give me an advantage against those annoying beasts and fiends but i'm a bit hesitant...
You see i've use my mega tundrus/magnus on these guys and they always dodge my charge attacks so i use my normal attacks on them instead.
my question is how does callahan fair against these annoying critters? and how do you use it to great effect? Charge attack or normal attack.

as always thnx bopp for the quick answer.
follow up question: does anyone use callahan exclusively to dispose of these guys (fiends and beasts)? do you have any tricks or strategies that you use to make the callahan hit its intended target more efficiently? if there is can i ask what it is? otherwise i'm going to stick with my peircing antigua until i get Dark Briar Barrage (as suggested by bopp).
Devilites tend to dodge towards the way a bullet is shot (not away for whatever reason). So you loose a charge aimed at the middle of a crowd of devilites and a good amount of them will die.

A simple tactic, which applies to every monster in the game: wait for them to attack (or in this case dodge), don't get hit ofc and while they're in this second of 'recovery state' bw their attacks, attack them.
But as Bopp said, get a DBB. W/ max ctr+dmg you can pretty much rekt anything which isn't strong vs pierc dmg.
Side note: not so much against beasts, but vs fiends I still find the blitz the best pierc gun.

When playing solo you take more of a monster's HP (percent wise) and when a monster loses a large amount of HP they are likely to flinch you can use a Callahan to flinch a beast or fiend then switch to a plague/blitz, then switch back and flinch them again. Just go back and forth till everything is dead.
The charge projectile is definitely slower than the regular projectile. If you wait for your target to initiate its attack sequence, then you can use the charge attack, which rips through them. But if the monsters are all in different stages of their AI sequence, then a bunch will dodge, so you will miss them.
If you use regular attacks, then stun happens fairly often, but the damage might not be great. I haven't recomputed it since the gunner update.
Although you didn't ask exactly this question, there is one weapon that is good at clearing fiends safely and great at clearing beasts quickly: Dark Briar Barrage.