Trap assassins usually has the easiest time to get max resist because they have the flexibility to either use Burst of Speed or Fade. You also have the option to wear the Natalya set since it gives loads of resistance and you could also socket the armor with resistance runes which could boost your resistance further. The only problem with using the Natalya set for a trapper is that you don't put the set to full use (it has 30% damage reduction and 14% dual leech), but aside from that, it's not a bad choice.
Max fireblast then lightning sentry and its synergies. 1 point into mind blast and burst of speed/fade. This build should be able to take on most enemies in hell while a merc can kill lightning and fire immune creatures.