I can't comment on where to put skills, but my brother has a trapsin and it blows through most of hell rather easily and even can take down bosses by itself. I believe the merc he has is an act 2 merc that wears dream helm that cuts down the lightning resist on enemies (lvl 20 holy shock). He does not have infinity on him yet I don't think. I didn't believe the holy shock helped until I saw it in action against a boss that was lightning resist and he broke it with his merc and the boss didn't stand a chance against us after that. So, it does work very well and trash falls very very fast.
And from the experience, it appears that +/- items do work with the trapsin. We can't be for sure on that, but it appears too. Again though, testing hasn't really been done, but I believe there is a thread here somewhere that states that it does help.
Either later today or sometime this weekend I can let you know what he wears as far as gear. It is high level gear, but maybe he'll drop by on here or maybe I'll talk to him and let you know what he used up to his lvl 95+ gear.
Start collecting Topaz's