Scáthach vs Tamamo Lancer

Scathach is a NP nuker with the 2nd hardest hitting NP in the game behind Jalter and the strongest NP if the enemy is divine/undead being the only servant in the game able to push her NP above 100k neutral damage without support at NP1. Scathach's guaranteed stun on her NP can be invaluable if timed properly as well. By contrast Tamamo doesn't do insane damage with her NP but it is much more spamable and thus a more consistent option. Tamamo is also able to delay enemy NPs a turn with the charm on her second skill(which I believe is guaranteed) while also giving a hefty 30% def down which stacked with her charisma skill means she can provide an impressive 50% attack buff to the whole party opening up options as a support role that Scathach simply doesn't have.

All in all Scathach will certainly do the job if you're just looking at rolling from a gameplay perspective and along that line leaving personal taste out of it Merlin is absolutely the best servant to save for as he is game breakingly strong.

Out of Tamamo Lancer, caster, Ishtar and Merlin, who do you like more?

For gameplay, I would say since you already have Scathach who is a top tier ST lancer, you don't really need Tamamo too, so it'd be more worthwhile for you to save for Merlin or whoever else you need more. Having Scathach means that you will never really NEED another ST lancer, even though both Tamamo and Bryn are better in certain situations.

To actually address your question though, Tamamo is a very good ST lancer with anti-male capability. Here are some things to contrast:

Scathach:
Pros:
- Has a dodge, along with crit star absorption and crit damage buff
- Third skill doubles her damage against divine/undead enemies. MANY archer bosses in the game are divine, so as an archer boss killer, her NP dishes out the maximum damage, very likely to one-shot them.
- Even if enemy isn't one-shoted by her NP, it also has a high stun chance.
- Has a targetable quick buff. Unless you have Jack on your team to generate stars for her, you will likely want to use this on her, so I can't say her team support is that great.

Cons:
- The major con with her is that she has the lowest NP gen of any SSR lancer. Her NP is amazing, but she can't consistently fire it off.

Overall, Scathach is a boss-killer with high NP burst damage, who also performs well as a crit DPS as long as she has a support to generate stars for her.

Tamamo:
Pros:
- Good team support: Has a party-wide attack buff that also buffs star gen for male allies, plus her second skill charms and lowers defence of one enemy.
- High NP gen
- Her anti-male NP can essentially hit for 1350% of her attack stat on male archers, and many archer bosses are male, so that's pretty good.
- Has invincibility

Cons:
- The only real cons I can think of for Tamamo are the demerits on her skills. Her charm charges the enemy's NP gauge, which imo isn't much of a problem since you will only be wanting to use it when their NP is charged anyway. Her invincibility stuns her for the next turn, but its manageable. At worst you have one dead turn, really not a big deal.

Overall, Tamamo is more of an NP spammer with more consistently high damage as opposed to Scathach's burst damage. She also functions better as a support.

Personally, I favour Tamamo, but like I said, Scathach isn't noticeably worse than her, and it'd be more in your interest to save your quartz for something else that you really need, unless you like Tamamo a lot.