Attacking creatures with equal (or higher) power to the blocking creatures defense will cause the blocking creature to die
Theres a reason this card is banned in Legacy and restricted in Vintage
The only reasons it isn't higher are because it requires a costly six mana each turn to pull off, eating into resources you could be spending winning, and because needing two colours in your colour identity limits the decks it could fit into
Nani gives you more flexibility than the higher cost Pluto Determined Defender since you wont have to rely on having another Pluto in play to get her onto the field, and effectively she does the same thing (Plutos healing ability is largely overrated and unlikely to make a huge difference in actual gameplay)
Stax, sometimes called Prison, is a control deck that wins the game by grinding your opponents out of resources
For one mana, Ragavan, Nimble Pilferer is a one mana Red 2/1 with a dash cost of two