1. Tell us what you have and what you can run.
2. You can field half your army in reserves.
If he's not running early warning or shyfire than the unit is vulnerable to reserves.
It's hard to suggest what to do since I only know half his list.
I think oblits deep striking would be a good ideal. Maybe units of one each. They deep strike and heavy flame pathfinders out of the bush.
Take bikers in reserve with lord that have burninng brand flame the blob. Make sure you get the side where you are killind the most suits.
Take cultist and have them field first. Plasma and melta would be wasted shots on theses guys.
Helldrake comes in and flame them really good and vector strke which is random wound allocation.
I almost want to say take daemons allies and run daemonettes, seekers, or hounds. Thinking about it run hounds and take a herald of khorne with the grimorie of true names. The unit will have a +3 invulnerable save. Take a caster and hope for endurance. If you cast endurance on the unit it will laugh off the bomb and can kill it eventually. The hit an run will help the hounds since they will keep getting furious charge and +1 attack. If you take loci than they can keep getting hatred or +2 on the charge lol on the hit and run. He can't escape the hounds with fleet, move through cover, and 12" of movement.
Don't challenge O'Shova with your herald. You will only have a +5 invulnerable on your herald. Taking the herald prevent you from taking another herald bearer of the true names. Unless you run a greater daemon. I don't like to do so since any model giviving +3 invulnerable saves becomes a huge target.
Again go for +3 invulnerable saving endurance hounds it will fix the problem. The unit with herald is half of what he is spending.
Also the unit does not have split fire. Target lock again does not give split fire. He can shoot seperately with the model with taarget lock. It doesn't allow him to pick and choose who can split fire and how many in his unit.