Overview: What? Why?

The Melee Engineer, or as I prefer to call it "Assault Engineer", is an engineer build that specilises in closing quickly towards the enemy and engaging the enemy in melee. It may sound crazy since ArenaNet didn't actually provide the Engineer with a melee weapon set, but not only is it doable, it's surprisingly effective.

The primary reason for its effectiveness is due to the much larger selection of mob control skills that Engineers have over seemingly every other class. Almost every one of your abilities can be made into an effective mob control skill using the appropriate traits. The vast majority of this mob control comes from judicious application of knockback, knockdown, stun and confusions. Add to this, certain builds of Assault Engineers have access to at least two blocking skills for any given situation and you end up with a build that can easily scrap in melee with the best of them.

The second reason is that the Armor gap between armor types is not nearly as large as people may think. The exact math will follow, but in essence from Medium to Heavy, you are looking at a 3.4% difference (even less depending on the build).

Build

The core of the build is the Tool Kit skill. With it, you have access to the following skills:
  • Smack > Whack > Thwack
  • Box of Nails
  • Prybar
  • Gearshield
  • Magnet


In addition, it is highly recommended that you take the following:
  • Healing Turret
  • Slick Shoes/Rocket Boots
  • Personal Battering Ram
  • Pistol/Shield Weapon Set


Traits:
  • Invention - 30 points - Stabilized Armor, Elite Supplies, Reinforced Shield
  • Tools - 20+ points - Speedy Gadgets, Power Wrench


For the rest of your Trait points and the matter of your Elite, I personally went with Alchemy and Elixir X. This gives me access to Fast Acting Elixirs (20% cooldown reduction) and another mob control skill, but I can see Supply Drop being equally as effective.

Core Mechanic/Playstyle
At the start of each battle, you will be closing distance to the enemy either by using Slick Shoes' speed bonus or by "exploiting" a hidden feature of Rocket Boots. This hidden feature is Rocket Boot's Rocket Kick skill which actually will charge you 300 distance towards the enemy you have targeted. This charge is especially handy after you've knocked an enemy back with Personal Battering Ram (simply run one step then Rocket Kick for a one-two whammy).

While engaged in melee, your primary methods of mob control will be through constantly knocking them down. Each of the following skills (from the core build) has a knockdown type effect.

Knockdown:
  • Slick Shoes: Run into the enemy so you overlap with them to force a knockdown


Knockback:
  • Battering Ram
  • Magnetic Shield > Magnetic Inversion


Pull:
  • Magnet


In addition to the above, you also have access to two blocking skills and a reflection skill:
Blocking:
  • Gear Shield
  • Static Shield


Reflection:
  • Magnetic Shield


You'll note that the trait choices above are primarily focused around reducing the cooldowns of all your pertinent skills. Altogether you should see all of your mob control skills see a 20% reduction in cooldown.

On top of the above, I recommend a judicious use of your various debuff skills like Confuse (from Prybar and Static Shot) and Poison (from Poison Shot). You may also find that Static shot is a great initial attention grabbing skill.

The Math

The basics of damage is:

Damage = Attack - Armor

The Attack value will vary with criticals and buffs, but it can essentially be thought of as the combined damage output of an attacker's skills, criticals, and buffs. Under normalised conditions, it can be treated as a static value. Condition damage isn't included above as it is directly deducted from the Defender's health.

Armor = Defense + Toughness

Looking at this formula, you can draw two conclusions.

First, vulnerability is a whole lot more powerful than it might appear. As it is applied as a direct reduction of the target's Armor. This is essentially an extra multiplicative bonus to damage instead of the normal additive bonuses you get from just stacking Power or Critical Chance.

Second, the true difference between Heavy and Medium Armor is actually very small. This is because, from Medium to Heavy, you will only see a 6.8% difference. A difference that I gathered from randomly sampling armors of equal level and same slot. This is also because Defense and Toughness has an Additive relationship. So the real difference is closer to 3.4% (less if properly built).

PVP
I have not tried the above build for PVP, but I can't see it doing too poorly. You have every normal advantage of the conventional melee classes in addition to just as many tricks (if not more because you're an Engineer). In addition, being able to apply so many types of debuffs has to count for something.

If any of you want to take some time out of leveling and exploring (where I'm at) to try this build in PVP, let me know how it works. I imagine you would probably want to move some points out of Invention in order to get access to more burst damage. For that I recommend going more heavily into Tools (for the speed boost and crit damage increase) and Firearm (for the crit chance increase).