Help for fodder

Here are questions you will want to ask yourself:
- How much will you use L.Lucina? Fir obviously you will use a lot because you're merging
- How much would you use Karla?
- Do you have non-combat attachment to Karla or your fodder-to candidates?
- Do you want to play for one-shots with Fir? I feel like that's what you'll end up doing if you don't use desperation on her.
- Can you one-shot the targets that you're trying to double with L.Lucina? Her atk is quite solid with her weapon effect and these super-fast swords can potentially fall in one hit due to weapon triangle.

If you're trying to figure out exactly how much speed you need, I wrote an analysis on the recent spd powercreep. You can find it through my profile.

That's a tough question I think you'll have to answer yourself. I'll give some guidelines on how I approach this.
1. Do you use the unit your about to fodder off? If you use them regularely, and don't have a good replacement for the role they cover then it might not be a good idea.
2. Do you like the unit/who is your favourite unit? Personally my favourite character is Lucina, meaning I use her a crapton. So I always prioritize giving her skills that will benefit her, over any other unit.
3. How much would the skill improve the units your considering? If it doesn't help much and its a very exclusive skill it's probably not worth it. (Although it doesn't look like it would apply here)

Hope this helped and if you want any clarification on anything, please ask. Also, if you think you could improve upon my process and give me some pointers I would appreciate that as well.