User blog comment:Hitsuji Mamoru/FGO-style Servant Creation Room/@comment-28209103-20170127004532/@comment-27971189-20170127135426

They are not on same tier with Waver and Merlin.

Waver and Merlin still in danger if enemies keep targeting them somehow, especially critical blow from enemies with low hit count like Ozymandias and majority of Berserker. But them, with Ruler defensive capabilities against all classes but Avenger and Berserker, can be said totally fine as they have two Invincibility and 2000 HP Regen; Tamamo pretty much cover their weakness and solidify them as strongest Ruler if you have them.

With Hikaru confirm their attack won't stop as their HP already reduced significantly, they are definitely Servant you won't like to met in Challenge Quest like Nero Redux or Tower. You may need party of Avenger (J.Alter, Dantes, Gorgon) to truly damage them they became boss.