What the heck is a trapper? An Assassin that uses one of two elemental traps, either:

20 Fire Bomb
20 Wake of Fire
20 Wake of Inferno

OR

20 Shock Web
20 Charged Bolt Sentry
20 Lightning Sentry

PLUS:

1-3 Burst of Speed
20 Fade
1 in requisites for/and Death Sentry
1 Blade Fury
1 Claw Mastery
1 Weapon Block (If using dual claws or going for Shadow Master)

(You can change Fade into another skill to max if you wish to do so, like Shadow Master in ex; I say Fade because of the res boost and +1% DR per point.)

91 points. I could basically describe some of the differences between fire and lightning trappers:

With Fire:

Fire Bomb is a ranged attack with an AoE detonation point. Area is 4 yards.
Wake of Fire emits waves of fire that extend with every point in this skill.
Wake of Inferno is just that, a trap that executes one second of Inferno. This skill is best used on monsters that are often stationary or cannot move.

With Lightning:

Shock Web is more of a "ranged placement" skill that attacks monsters in its AoE every 0.9 seconds. Useful if you're taking out weak trash early on.
Charged Bolt Sentry releases charged bolts. Multiple hits can occur with the bolts, but the chances of it happening decrease as the bolts scatter outward.
Lightning Sentry fires a bolt of lightning. If facing narrow areas, monsters that are often stationary or cannot move, or monsters in a line, this could be an option for deployment.

But now, there are two options that I have added: Death Sentry and Blade Fury.

Blade Fury will take out monsters immune to your trap element.
Death Sentry will explode corpses whose monsters are in the explosion radius.

Weapon Block (or "claw block" as few will say) can block some things a normal shield cannot. Plus, Weapon Block is static, which means that blocking is not dependant on DEX when factoring chance. The chance you see on Weapon Block is guaranteed.

Trappers aren't for everyone, though, but if you want to give one a try, this will at least give you a general lowdown. You can also search this forum for extra advice on being a trapper.

You'll need some patience on being a trapper and you will be most often using your weapon and mana pots until you can get decent mana recovery and mana steal with your Blade Fury. Also, don't forget about the Eth rune, it greatly increases your chances to hit with your blades.

You will be looking for Increased Attack Speed modifiers when being a trapper. Trap placement is modified by your attack speed. So, the more IAS you have, the faster you can lay traps. Traps will be placed at your feet, so it is important to have IAS so you can lay down a trap and move.

When a boss executes a "source counterattack" where he will strike the attacker, he will go after the trap, so if you move out quickly, you won't be in his way. Traps only die if they run out of shots, so you can use a weak sentry as a kind of decoy to attack the boss while you gather your strategy with the group.

And finally, what I suggest for a decent trapper is enough STR/DEX to wear your claws, some boost in Energy if you have mana trouble early, but otherwise use the rest of your stat points in Vitality. A trapper that cannot survive well is a useless trapper, so if you die often, you've flawed somewhere in your build. (Rework your strategy or team up!) There are sources where you can gain boosts to your life, mainly rubies and blood-crafted rings. (Blood-craft rings are also a good source of +STR and life leech.) Charms with + to Life are a great find as well, especially if you play Hardcore (A bonus if you find one with resists or a + to skill tab).

Dual wielding claws is an advantage over claw-shield in many factors, since later claws can spawn + to skills, including +1-3 to the trap you use (I overlooked this one, thanks!). Claws can spawn with resistances - don't forget that.

I haven't tested this thoroughly, but I am led to believe that -% to enemy resistance might/might not work, although +% to skill damage does.

Anything else I've failed to mention here? I'm sure I did since it's early in the morning since I've typed this up!

EDIT: Extra suggestions added, 3/26/10 20:00 EST

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Go and terminate with extreme prejudice
Most people just take steps. I take dubsteps.

Put those 20 in a shadow instead. Much more useful. And only an idiot would pass up on claw block. Better claws than shields usually and the fixed ctb along with the ability to block elemental attacks makes it superior to claw/shield.

I do agree that weapon block can indeed block elemental attacks. But without a shield, you may need to play tricky on getting the resists up (not to mention absorb later in the game).

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Go and terminate with extreme prejudice
Most people just take steps. I take dubsteps.

resists are no trouble , you can even have more than enough with dual claws.
weapon block is one of the reasons assassins live . They had their hp buffed a bit this patch but still the claw block is great (!)

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'may the force be with you' - Yoda??

Claw block is definitely a requirement, and I would say that for a trapper, dual claw is a no-brainer anyway - you aren't going to find +3 traps +3 light sentry on a shield (at least, I don't think you are ).

Also, a shadow master can theoretically double your trap output, not to mention provide a strong meat shield and distraction. Finally, wouldn't points be better spent in Death Sentry than Charge Bolt or Shock Web? Or are you going for pure synergy damage?

shadows fuck up your traps, they get (your skill level/2) + (their skill level/2) iirc. thats for shdow warrior iirc, dont remember the masters one.

Probelm with them using traps is that there can only be 5 traps out at once. If a shadow casts one then one of your traps dissapears and the shadows traps are usualy weaker by a long way.

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And Batz, ofc you need to fully synergise the traps. Or you'll suck later on.

Yeah, even though having 2 minion tanks is nice, they won't stay alive too long if invested in very low... Fully synergize your traps and rely more on Mind Blast--it's very nice to have stuns (try to get FCR on your rings and a lot of other equip, assuming you are reaching the best IAS for your trap laying) and converted minions in many situations.

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-ISO- Moonstone Rubies, +6/-6 Psn/Lite Facets,
Hearts (pal/nec/dru/zon), Braaaains

I wasn't positive if Death Sentry provided synergies to the lightning traps. I had a vague mental picture that it didn't. However, in the interest of being a "support trapper" I figured disposing of corpses with high damage output may not be a bad option, regardless of synergistic effects.

I would figure that a support built is more looking for exactly that - support, not cranking out high damage. In that scenario, perhaps having higher level summonable tanks and mindblast would be better as far as support goes. Alas, I am not arguing, I'm just offering my opinion, inexperienced as it may be.

Also, I didn't think about the fact that shadows override your traps. Once again, my memory is shady but I feel like vanilla 1.09, shadows could cast traps in addition to yours; I haven't made a trapper in HU yet so I wasn't aware of the limitation.

What he is describing here is a normal trapper though, regardless what he is calling it.

If you want to be a pure trapper, you're not going to rely on anything else other than claw block and your traps. If you're going to support a group or at least yourself, then consider broadening your options.

Utter, I said earlier that I was just waking up. Perhaps I should change that topic header now that I'm more alert.

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Go and terminate with extreme prejudice
Most people just take steps. I take dubsteps.

batz: +3 traps, +3 wake of fire as well? for a fire trapper?

From all I do know, Charged Bolt Sentry is the only sentry whose projectiles do not pierce. But it's also the only sentry whose projectiles stack for extra damage if your enemy's close enough to it.

_________________
Go and terminate with extreme prejudice
Most people just take steps. I take dubsteps.

Inferno don't get the pierce afaik.

I've used wake of inferno and I've seen it hit multiple targets in a single spray. They have to be in a line to get hit though, so it's only useful for narrow areas. Useless when your enemy's moving too fast to get hit by it; in that case, wake of fire would work instead.

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Go and terminate with extreme prejudice
Most people just take steps. I take dubsteps.

Ah ok, you were talking about that kind of pierce. I was talking about piercing enemies resistance. Aka -% enemy res.

I should've clarified that. But again you give me a detail I should add. I didn't know WoI didn't work with -% enemy res.

_________________
Go and terminate with extreme prejudice
Most people just take steps. I take dubsteps.