1. Have more life than your opponent
2. Switch power and life total
3. Kill opponent and gain back the life
The problem with Evra is that the opponent can just concede when you swing, leaving you at four if you can't pay to switch again.
More fun combos. Most very bad. Keep in mind that when you switch back you gain life equal to the difference.
Sanguine Bond - 8: Each opponent loses your life total-4 life.
Rhox Faithmender - 8: Your lifetotal becomes twice your lifetotal-4.
It retains buffs, so if she has 5 +1/+1 counters on when you switch, her base power becomes your life total, switching back will leave you 5 more life. Best utilized if you also swing with her.
Axis of Mortality - Switch your life during an opponent's end step. During your upkeep switch your opponent's life with your own 4.
Soul Conduit - Switch, then swap life with an opponent, leaving them at 4.
Not really a combo, but Blazing Hope becomes a quality removal spell when you can drop to four life easily.
Black so doesn't work if Evra is your commander, but:
Repay in Kind - Switch, then make everyone else's life total 4.
Tainted Sigil - Gain life equal to the difference between your life total and Evra's power when you activated her. Gain even more if you connect with her.
Tree of Redemption - more shenanigans.