There is no route which completely avoids hills, if you really are unable to manage any kind of elevation then the Camino might not be for you. Careful planning would be required using a good guidebook, website or app, to arrange alternative transport for the hilly sections. Remember, you can skip sections if you need to. The only rule (if you wanted the Certificado or Compostela is that you walk the last 100km and prove it by collecting two stamps a day in your pilgrim passport). 

The flattest final stretch would probably be the Camino Portugués. If starting at Tui or Valença or Baiona, you'd encounter fewer hills than any of the other 100km start points. 

For basic profile maps on all routes:
http://caminodesantiago.consumer.es/

Please note that the CSJ has the St Christopher Fund to help with expenses for a companion to travel with elderly or frail pilgrims who need this kind of support. Contact us for more details if you think that this may help you.