Sniper Shot is both -20% atk and def, which extremely strong and a huge debuff but since the recent cap of -50% and with the party I mentioned above, it is not as amazing anymore since you can't break the game with like -70% atk down anymore and because every other hime in the above example brings in a strong -20% debuff. However, you only have half of the SSRs I've listed as the other half aren't even in game yet. So Sniper Shot will play a big role in helping until, if you ever, get the other SSR himes. Regardless, by the time SSR Artemis is released, if you're still playing by then, you should have all of the regular souls unlocked already.
For ex skills, it's not a matter of what goes well with Mordred but rather what you need to fill in with your current team composition. If you need an a-frame defense down, then you get Ambush, if you need an a-frame atk down, then you get Trial by Jury, if you need b-frame atk or def down, then you get Sniper Shot, etc etc. I don't know who else you have so I can't really say anything. Priority soul is still Mordred. Who you want to prioritize after can change depending on what SR's you have since you haven't mentioned them.