Why is Water Blademaster (Rana) your girl?
Unlike many champs, even many Nat 5's, Rana (as well as other BMs) doesn't rely on a huge health pool as her primary form of defence. Dance of Death deals an immense amount of damage to a random pool of enemies, providing both a good AOE and single target skill option, but it also gives her several seconds of immunity to all damage. While playing auto, this can get you lucky. While playing manual, this can save your booty.
My cup overfloweth (with damage).
Not having to worry about her health pool or defense allows us to stack an insane amount of Attack %, Crit %, Crit Chance, and Attack Power on Water Blademaster (Rana) . Many DPS units in DHC and other comparable games serve as glass cannons—but because Rana can give us invulnerability as well as mobility with her first skill, Blade Rush , she can deal extreme forms of damage against all elemental types.
Being hard to kill has its perks.
This is where it all comes together. Use Water Blademaster (Rana) for everything, unless you have a champion that is absolutely better than her. Use her when her element (water) is strong against her enemy (fire), use her when there is a neutral damage/defense ratio, and use her when she is weak to her enemies. Her elusiveness will negate any disadvantage she has against nature opponents, and her raw ability to deal damage will make up for the elemental disadvantage.
If you have someone better, use 'em. If you don't, use Rana.