Just theorycrafting here, obviously, but my instinct would be using range to whittle and/or remove their key units (Daemon Princes, anything that can shoot etc.) and whittle their other units, and then Incubi, Grotesques, Hellions and such to finish them off.
We definitely can't beat them in melee in a fair fight, so our aim has to be to ensure that fights are never fair.
Something interesting to note is that (as far as I can see) they have very little in the way of 'fight first' mechanics. Thus, any melee units that can strike first in their turn (Archon with Ancient Evil, Incubi, Cult of Strife etc.) could make things quite awkward for them - especially given their lack of shooting. Cursed Blade could also be interesting with their mortal wounds ability.
We'll have to be careful with transports because WE have a lot of bonuses to movement and charge distance - and the last thing we want is for a transport to be surrounded and destroyed, likely auto-killing the unit inside.
Their blood-fuelled abilities (seriously, why is it that every other army can have active abilities based around killing models but it's impossible for us to have anything but the Table of Boredom?
) don't seem hugely concerning, as most of them are just 'kill more', and I suspect that they'll blend most of our units regardless.
The only one that really concerns me is the one that auto-revives Angron with half his wounds remaining. Seems a bit stupid, given how inexpensive it is.