Hardest part?
Probably hitting your seed...and I don't mean watermelon seeds xD
Because unlike in 4th Gen. there's no way of knowing it, except using Pokes with Seismic Toss/Night Shade with certain levels against the target.
Example: You'll know Zekrom has 31 IV in HP if it survives 2 Night Shades or 2 Seismic Tosses from a Level 87 attacker, although it could also mean Zekrom has 30 IVs in HP.
Because Zekrom will have 175 HP stat if it has 30/31 IV, at level 50 of course.
This will help BTW, but there's a chance the target has the wrong nature.
And we have to know our adjacents too. It's better if the adjacents don't have 30/31 IVs in HP.
Or probably analyzing False Swipe damage (when it doesn't reach 1 HP), or the damage received from the target poke.
Also hard is to get the right nature(s) and ability. For now, we'll ignore the gender.
BTW, I of course used a syncher...which happens to be an UT Mew with flawless IVs. I let it hold smoke ball to escape.