I found this boss battle to be one of the hardest in the game - however the best tactics for myself was to string together hard hitting charged combo's with the Eryx ability.
Whenever possible Angel pull & Angel glide to get in close to either his side or eye, chain a powerful combo together then glide out to avoid his swinging attacks.
Once he is down, Demon pull out Lilath and start to attack her until the Spawn recovers, rinse and repeat this process.