You have a few options.
1) Get an assist lead and maintain it for the entire match
2) Run an initial assist%+ build, which helps shortcut to step 1
3) Don't miss
Steps 1 and 3 are what I prefer. I run an EX build and don't whiff often. When I do miss an attack, I try to make sure it's not something I can be punished for. By maintaining an assist lead against most of my opponents, I don't get assist punished. If they do try to assist punish me, I assist change (if I don't have 2 bars, otherwise I just assist lock them) and lock the assist with Sacred Cross.
That being said, it takes a high skill level to deal with assist punishment on a character as punishable as Cecil.