In the Holy Grail War, Savior is a massive roadblock for just about anyone who actively seeks the Holy Grail. When fighting at full power, He can battle all six other Servants at once; however, the sheer strain it puts on both Him and Sakura prevents them from doing so all the time, forcing them to be tactical about things. It helps that neither of them
actively want the Holy Grail; YHWH sees it for
and Sakura trusts His judgment when He tells her that it isn't worth chasing after. They prioritize, instead, making sure that the War does not kill anyone that doesn't need to die. If no one seems inclined to, say, attack innocents, then they are perfectly willing to sit off to the side and watch the War go by.
...however, there may still come a time where they have to fight. In that case, Sakura prefers hit-and-run tactics, as she isn't remotely suited for combat;
YHWH is much more capable of fighting on even ground, but cannot do so for very long without exhausting Sakura. The only time in which Savior can be killed is when
YHWH is inactive; Sakura may pack a punch with Mana Burst (Heaven), but she is still completely mortal and her physical attributes have not been improved by her Pseudo-Servant status at all. If
YHWH's manifestation has exhausted her... she's a sitting duck. And in the state He is in now as a Divine Spirit,
YHWH cannot resurrect the dead.
All in all, Savior is a
Servant-killer, and
— often due to the two's mutual pacifistic nature
— fares poorly against Masters that can actually put up a fight. They are still the most intimidating duo of any Holy Grail War, and, if Sakura were any stronger, would be able to give a
Grand Servant a genuine challenge.