Please help Britons and Celts destroy me

Please help I have been playing along time but when ever I come across the Britons or the Celts I always get destroyed keep in mind this is the AI the Britons either bring mass longbow man and trebuchets or the Celts bring mass scorpions the only thing I can kind of come up with is just throwing rams into the longbow men to take some of the damage and putting skirmishers behind that but they always bring pikemen or something along those lines that just destroy the rams please help