The problem I see with traps is that for traps to do the kind of damage you need to do in Hell, you need to max out 3-4 skills. This means that unless you are rich, it's hard to pull off a viable Trapper that does 2 kinds of damage. The trapper's lightning traps are arguebly more powerful than the Sorc's entire lightning skill tree, but specializing in one element leaves you helpless to Lightning immunes. If you want to go a trapper, I am of the opinion that you should focus on building up a Merc, or joining games that have fighter type characters in them.
Here are some numbers for you:
Lightning Sentry, slvl 20, with all synergies slvl 20 does 8-2574 damage, shoots ten times. To get this damage, you must max 4 skills. I consider it the most viable of all traps.
Death Sentry, slvl20, with all synergies maxed, explodes corpses/shoots lightning 11 times. Corpse explosions will deal 40%-80% of the corpses life to offending monsters in a radius of 9.6 yards. When there are no corpses handy, it will do 3-1040 lightning damage.
Wake of Fire, which never did that much damage, is one of my favorite traps, in that it looks cool -_- . There's a thread about this topic that was posted fairly recently by yours truly.
Click here
A martial artist has no problem with immunes, because it can summon Meteors, Chain Lightning, and um Ice Bolts, in addition to their physical damage. How viable they are depends on what equipment you have, as is true for any fighter type.