Because 50 is half of 100, you'll need 8 EVs to raise a stat by 1 point. Natures still multiply the stat by 1.1 (rounded down).
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There are a bunch of other ways to to make your Pokemon more difficult to knock out, like screens, protect, snarl, wide guard, heal pulse, "pinch" berries (Figy berry, Wiki berry, etc), damage-reducing berries (like Shuca berry), focus sash, assault vest, or intimidate.