I'm going to say that you should use DC's guidline from GM section. For example, versus 1st level goblin you should use a high 1st level DC wich is 14. Versus Darcus it is better to use 3rd level severe DC wich is 18 (since he is a 3d level boss).
I have a question about disarm. For example I have 1 lvl paladin +4 str trained in athletics (+5 total). I'm going to disarm 1st lvl goblin (DC 15 Reflex). So I have a 55% chance to succeed (10-20 on roll) and only 5% chance to crit (just natural 20).
On success I do not disarm goblin, instead just weaken his grasp (wich is giving me +2 bonus on next attempt). But my next attempt to disarm suffers -5 penalty (wich is results in no chance to disarm the target at all). Can I use -4 penalty for agile? Is unarmed (free hand) works as agile?
If I'm a ranger and i'm hunting the target, can I use -3 penalty on my second disarm attempt?
Multiple attak penalty may looks "ok" with other combat maneuvers (since success is actualy success), but disarm looks useless with this penalty as it almost can't result in actual benefit (at least for party with single athletic character).