It means gain +50 fifty life from +life abilities.
e.g +(X) life per damage as such found on kenny and others, or defeat: +(X) life like lola/kusuri have, or straight +(X) life like diana. This mission doesn't require you use rescue and should be fairly easy.
You can use any + life card to gain life (because they don't specify to use rescue in this one)
+ life per damage
stop + life
defeat + life
courage + life
+1 life per turn (bridget)
in rescue, there are mark, anita, krash, suzie and aurora
It means you have to accumulate 50 Life points in a battle by using any card that has a + Life ability in it.
You could opt to use the Junggo clan to finish this mission fast.
Or another practical way of finishing this is, by utilizing the Leader Bridget.