Saki is more than worth the DC in my opinion. While she isn't needed for every map, she is useful in a fair number and can help make plenty of maps easier or at least faster.

That said, another unit to consider if you have it is Solano (the plat summoner, though the black one likely also works). The reason is that while her range (like all ranged units) is reduced, her splash damage radius is not. Thus if you have a unit blocking the top ogre and have her attacking it, she will also hit the middle lane for a fair amount of damage. At least until the top ogre dies. Then you should just withdraw her for UP.

Anyway, the map is probably doable without blacks, but to kill the middle lane you'll almost certainly need at least some form of melee slot ranged damage. Whether it comes from Saki, some other ninja, a magic fencer, or some other random unit that can do so. While Olivie is useful, you can also make due with either a string of particularly tanky units, or some rogues and some luck.