You can activate Loxodon Lifechanter's last ability after it has entered the battlefield but before its first ability has resolved.

You choose whether to change your life total and, if you choose to do so, you set your life total to the appropriate number while Loxodon Lifechanter's triggered ability is resolving. No player may take any actions between the time you choose to set your life total and the time your life total changes.

For your life total to become a certain number, you gain or lose the corresponding amount of life. For example, if your life total is 4 when the ability resolves and your creatures have a total of 6 toughness, the ability will cause you to gain 2 life; alternatively, if your life total is 10 when the ability resolves, it will cause you to lose 4 life if you choose to change your life total. Other cards that interact with life gain or life loss will interact with this effect accordingly.

If you set your life total to become equal to the toughness of creatures you control, this happens once. If the toughness of your creatures changes later, it won't affect your life total.

The value of X is determined only as Loxodon Lifechanter's last ability begins to resolve. It won't change later in the turn if your life total changes.