Servant to kill ruler

If you want a Ruler Killer, go for Lu Bu as his NP is for single target so it hits harder than Kiyo's AoE NP.

...use Dantes?

Not counting the likes of Robin Hood or Euryale who deal bonus damage under circumstances, Lubu has the strongest single target NP out of all the 1-3 star servants. Use him alongside Dantes to take out Amakusa before he can shoot off his NP. Jeanne on her own won't be a threat unless you're nearly wiped out.

Which can definitely happen as you'll have to deal through a trio of Caster Wyverns and a very power Rider Wyvern before you can fight the two Rulers so having Kiyohime nuke the Caster Wyverns is a good idea.

The best way to deal with the Rider Wyvern is with Ryougi Shiki since she has an invulnerability pierce move that negates the Wyvern's invulnerability skill. If you don't have her then you'll have to make do with whatever assassins you have.

Keep Dante in the back so that you can keep him healthy for the final wave.

Good luck.

To add to what FollishPenguin said, if you don't have Lu Bu levelled, but you do have Robin Hood or Euryale at a high level, you may want to take them and try to overcharge their NPs. They'll still do only half damage, but considering how hard their NPs hit, that may still be substantial.

If you were able to NP 5 Assassin Shiki, teaming her with support Dantes and another star generator may allow you to Crit Shiro to death, assuming you survive the first wave of casters that is...Maybe start with a rider and swap them out for one of the assassins in Wave 2.