I have always found he wins in two main ways;
Forcing the opponent to activate better models before me; I crank 10+ activations a turn and the first are rats walking.
The second is making the nasty enemies insignificant, which is hard late game, but depends on how many activations I can do.
Pandora, Sonnia and Witchling Stalkers are threats to Hamelin IMHO, the other stuff, not so much, he is a toolbox which is why he is hard.
I would much prefer they change his rule to nerf some of his stuff, but allow him more options force-wise.
Anyway, I will stop harping on about. I do love the Malifaux system . . . I just need a new faction