How to find players at minimum IL?

[Niwashi]

So all trials (50 lvl(hard)) have same problems?

The same as Castrum and Praetorium? No. Those were designed to be done by players who had just reached 50 and hadn't even begun gearing up for level 50 content yet (since none of the other level 50 content is even unlocked until they've been completed). So it's balanced for players with item levels still in the 40's. That's what prevents level sync from working properly on those. Their designed item level is simply too far away from the item level that they sync to. It's that distance between designed and synced levels that trivializes them.

Leviathan, on the other hand, was designed for players who had already spent a couple patch cycles gearing up on level 50 content and are well on their way into a third. While level sync brings players to the same level as it does in Castrum & Praetorium, the fight itself is nowhere near the same level. You'll see a real fight there, not perhaps an overly difficult one, but at least enough to provide some excitement, a few cool mechanics, and a sense that it could fit into its story lead-up.