-atk/+def Cordelia

My personal rule as an f2p is always fodder unless none of the skills are needed. The stat increase from a single merge is just not significant enough to outweigh the benefits of passing rare skills. Cordelia's skills of note for inheritance are her Brave Lance and Galeforce. If you have a unit that needs one of these then definitely fodder. (If no one needs those skills, you can also use her for TA fodder if you're desperate for it, but I wouldn't recommend it, wait for Roy to show up instead.) If you have no units who want these skills and no plans to build any who would, then you can go ahead and merge if you want, but honestly I might just sit on her and wait anyway, because you never know when you might need an extra brave lance.

If you want a +10 Cordelia in your barracks one day, then merge.

If you want to make someone stronger with Brave Lance+ and Galeforce, fodder!