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So far we know that:

After every fight Orb gains 1 Ultraman Card.

Juggler gains the monster's Kaiju Card.

What seems to happen is that the more powerful card or cards Juggler uses, the more powerful the monster he can summon.

Based off these two facts and my speculation. I think Juggler is trying to acquire enough of the strong Kaiju Cards to summon something extremely power, but has to do so without giving Orb too many Ultraman Cards in case Orb could have enough power to stop him.

If Juggler plays his cards right (Pun not intended), he could cause a cycle that allows him to summon more powerful monsters at a faster rate than Orb can keep up.