Engineer

Tactics

 * don't detonate on contact with walls, although they detonate after 4 bounces (or on contact with an enemy). This means you can bounce them off of walls to hit enemies behind you or from odd angles in a vertical shaft.
 * Syringes will speed up the time between the engineer being able to shoot again.
 * In terms of items, any on-hit items are amazing, since the Engineer shoots 3 grenades with his basic attack.
 * The Engineer can lay mines on-the-go, making them an excellent tool for "kiting" (attacking enemies while you flee to stay out of range of their attacks).
 * Sometimes laying a mine or two near your will redirect monster aggro. This is a great tactic you can use to distract large amounts of mobs, including bosses; this is especially good against Providence, as you can place a turret, stand back and batter him while he hacks away at your turret.
 * If you're quick, you can jump off a ladder/rope and drop a mine before grabbing on again, allowing for safe damage. The same can be done for your . They will typically fall to the ground below, but not always. This also works off ledges, if you hop out, drop a mine/turret, then quickly change direction to land back on the ledge. It is possible with no jump or move speed boosts, but much safer with them.
 * The mines, when detonated, is treated as an on hit, making on hit and chance on hit items to be able to proc with it.
 * Spirit can make mines very hard to use. Any fast-moving enemy can trigger a mine, but move past it before the detonation. This can cause an entire field to go off with no damage done. Jump over the enemy so it changes direction on top of the mine; this should provide enough time for detonation.
 * Improperly clustering mines can also reduce their effectiveness. An enemy hit by a blast may be knocked past the next mine, causing it to detonate without dealing damage.
 * Has a good range and tracking, easily able to hit enemies in hard-to-reach places. This saves the Engineer a lot of travel.
 * You can safely use in a group of enemies by laying two mines, and jumping while activating the skill. This will give you enough time to resume running at a safe distance.
 * Stops all horizontal movement of the Engineer. This is mostly detrimental, requiring time to fire it while remaining safe. The Engineer can jump at any time while using it, so vertical dodging can work well with Hopoo Feathers or Photon Jetpack.  It can be useful, too, if performing tricky jumps, by using it to stop you from overshooting a platform.  Try it when getting the Spirit Artifact in the Temple.
 * Stack on syringes to reduce the time it takes for the harpoons to fire, reducing the time you're immobile.
 * Their attack speed scales with the engineer's current attack speed, including attack speed modifier items.
 * They are not invulnerable, although they sometimes seem extremely tough and other times die very quickly. Putting them in a slightly hard-to-reach spot, such as just past a gap in platforms, will allow them to aggro without actually taking damage. Try using them as a meat (metal?) shield with mines and grenades stunning enemies.
 * Be VERY careful with electric enemies (elites, vagrants and worms) as the Turrets will CONDUCT electricity, increasing the range of the enemy electricity and possibly hitting you in the process (if you've just dropped them, for instance).
 * Turrets can be stacked with mines to aggro worms and vagrants quite effectively.
 * Turrets sometimes act as a decoy and draw enemies towards it, they seem to have a aggro priority equal to the player, making them gain the player's aggro if the enemies lose it.
 * Alien Head will generate Turrets and presumably Mines faster. However, Gigantic Amethyst (item that resets cooldowns) will not give you an instant turret or mine when it's used. The Wicked Ring does not appear to provide mines or turrets any faster while obtaining critical hits.
 * Engineer benefits greatly from items that give him additional time in the air. Thermal Harpoons can be fired safely while airborne and staying in the air for a second or two will often cause enemies to clump up beneath you, making for maximum mine efficiency. Don't forget that Mines can be dropped while airborne as well! An effective item for doing this is the Photon Jetpack as you can not only keep airborne longer but also maintain some forward momentum as well.
 * Dynamite Plunger is a fantastic item for the engineer, especially at low levels, against worms/elite worms/cremator (and most bosses generally), as they can 1shot most of them with a good pile of dynamite and the right timing.
 * Note that items such as Brilliant Behemoth will NOT affect your turrets. They will just continue to fire normal bullets.
 * Engineer benefits greatly from items that give him additional time in the air. Thermal Harpoons can be fired safely while airborne and staying in the air for a second or two will often cause enemies to clump up beneath you, making for maximum mine efficiency. Don't forget that Mines can be dropped while airborne as well! An effective item for doing this is the Photon Jetpack as you can not only keep airborne longer but also maintain some forward momentum as well.
 * Dynamite Plunger is a fantastic item for the engineer, especially at low levels, against worms/elite worms/cremator (and most bosses generally), as they can 1shot most of them with a good pile of dynamite and the right timing.
 * Note that items such as Brilliant Behemoth will NOT affect your turrets. They will just continue to fire normal bullets.

Achievements

 * Sticky Bomb: Detonate 15 Bounding Mines within 5 seconds
 * Concussion Grenade: Kill a boss in 15 seconds or less

History
Engineer