Rulings

Each time Kelsien's last ability resolves, it creates a separate delayed triggered ability. If you untap Kelsien and activate its last ability multiple times in one turn, they'll each trigger when the appropriate creature or creatures die.
If the target of Kelsien's last ability dies before the ability resolves, you won't get an experience counter.
If Kelsien leaves the battlefield, you keep any experience counters you've received.