Hamelin is an interesting boss. Because of the way he spawns adds, you can kill him almost exponentially faster as your damage increases.
I think what happens is that when he spawns rats, he keeps spawning them until a certain number are on the ground. So if you do enough damage, you can keep him summoning indefinitely.
And the rate of summoning seems to increase the faster you kill those rats, so maybe it’s like he summons at 3x rate till Y rats are on the ground, then 2x rate till there’s 2Y rats, and then 1x rate till 3Y rats… or something like that.
I’ve noted that when pushing high with Leapquake, you can’t do enough damage to make this happen without both a power and a channeling pylon (as a result, Hamelin is a worse boss than Saxtris or Binder)
Luckily, Rend doesn’t have that issue!
Both the CoE/Crimson and the F+R / MoC setups are good. F+R does a little more damage in general, while Crimson is a bit more fluid to play and also gets a bit of damage boost from channeling pylons.
If you have good gear for both setups, then try both and just pick whichever feels more natural to you.