This 5th level spell lets you target 1 object up to 80 bulk...and move it up to 20 feet. Unlike other spells, I the word "unattended" is not included in the spell description or target text. What happens when a player targets a creature's armor? Can you target a worn or carried item? If you do, I assume RAI you should either make a spell attack vs reflex/fortitude resistance or the affected target(s) would get ref/fort saves vs caster's spell DC.
Hmm, that seems like an oversight to me.
My quick ruling on the field would have to be that because the Armor in question is part of a group of objects, namely the armor, character, clothing, gear etc... it is not a valid target.
This is not a water tight argument, as any object with multiple parts like say a wagon or similar man made object then becomes an invalid target. But the intent of the spell does seem to be for moving huge boulders and not the breastplate off of some schmuck, or definitely not the schmuck himself with all his gear. Especially not without any save mentioned anywhere in the spell.
What happens if you take a huge object and just place it on someone?
An 80 bulk object could be huge.
There is a DC 15 reflex save to escape falling people and other objects, is that the best we can do?
If I drop a giant boulder on you, you can save against the damage, but will you still be pinned by thev weight?
Can you move out if the way without having an action ready, or delaying?
Dropping it on the person would have the same effect of it falling on them, with the save accompanying that.
Placing the boulder on someone gets a lot more interesting, and a LOT less covered by the rules. As the GM you could apply a damage to the boulder and force the player to roll to hit them with a spell attack roll, but I'm not sure how you would figure out the damage that a multi-ton boulder would realistically do, especially if it lands square on a creature.
As for pinning/squishing a creature with that boulder, that may very well just have to be a case by case situation. It would make sense that that tactic would be a combat ender against most opponents, which means that as a GM you have to decide if you want to allow it. You could rule that the Boulder is simply too slow to "catch" an aware foe in a way that they would be trapped. After all it is travelling at roughly walking speed every turn. On the other hand if you allow players to sustain spells multiple times a turn, it could move 60 feet in that same turn, which isn't too slow realistically.
The CRB just doesn't account for that sort of tactic honestly.