[+2]: You gain 2 life. Put two +1/+1 counters on up to one target creature.
[−3]: Exile target creature. Its controller gains 2 life.
[−10]: Creatures you control gain flying and double strike until end of turn.
Ajani’s last ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won’t gain flying or double strike.
If the target creature is an illegal target by the time Ajani’s second ability tries to resolve, the ability doesn’t resolve. No player will gain life.
You may activate Ajani’s first ability without choosing any target. If you do, you will gain 2 life. However, if you do choose a target and the target creature is an illegal target by the time the ability tries to resolve, the ability doesn’t resolve. You won’t gain 2 life.