As with other siege battles, one of your opponents will 'defend' the battle, and you remove counters from it to defeat it by dealing damage to it, just like attacking a Planeswalker
Then attacking into this for basically free damage (-300), triggering it to Special Summon Ceruli again, gaining another extra effect and enabling another attack into Ceruli for free damage (-100)
That way it wouldn't become a bigger problem even if it gains 2 effects from the Ritual Djinns
With a little less than a month before the set hits shelves, theres still plenty of time for the publisher to sneak out more spoilers
For the uninitiated Commander is a semi-casual format for Magic: the Gathering
I will write more about this service, technology, and development in a future blog post