For your life total to become 15, you gain or lose the appropriate amount of life. For example, if your life total is 4 when Form of the Dinosaur’s first ability resolves, it will cause you to gain 11 life; alternatively, if your life total is 40 when it resolves, it will cause you to lose 25 life. Other cards that interact with life gain or life loss will interact with this effect accordingly.

Form of the Dinosaur’s last ability isn’t optional. You have to pick a fight with an opponent’s creature if any are legal targets. If that creature leaves the battlefield after it becomes the target of Form of the Dinosaur’s ability but before it resolves, you won’t be dealt damage.

In a Two-Headed Giant game, Form of the Dinosaur’s first ability causes your team’s life total to become 15. Only you gain or lose life this way.