Cresselia for ultra league

I powered up one that ended up at 2495 CP, and I've really liked how it performs in the Ultra League. It's attack is low, but both it's defense and HP are high, so it can withstand a lot of punishment. I run it with psycho cut (for the higher EPS compared to confusion) as the quick move and aurora beam and moonblast for charge moves. This gives it a nice range of coverage, and the EPS from psycho cut allows it to use charge attacks pretty frequently. Unlocking the extra charge attack is a hefty investment, but even if you don't do that, I think it's still be a worthwhile Mon to use.

Get an awful IV Cresselia from trading, she could end up below 1.500 CP and be useful in Great too

Cresselia is tanky, hard to defeat, but easily countered. Teaming it with a Giratina disrupts a counter team.