I like it best in green decks that ramp out a bunch of 5+ cost threats, at which point Ugin lays waste to everything else (the ramp also helps play Ugin at a reasonable time)
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It can also protect your team from a board wipe or grant double strike to a creature, making this a card that just does everything an aggressive instant could want to do
The reason phasing is so confusing is because the way it works today is very different to how it did when it was first introduced in Mirage
This is great for a deck trying to go wide by generating a ton of small creatures