This is just a thought-build and would like feedback on the pros/cons folks see. The concept is a fairly tough - not a tank but can take a hit or two (via PRR, MRR & SR) - massive damage great crossbow build. A ballista.

I went with Drow for the Int, Dex, and Spell Resistance but it seems to have lost a lot on hit points, so it might be worth the switch to Human to conserve skill points and trade-off SR, +2 Int and +2 Dex in exchange for +2 Con? It will probably be more than that because the cost for Dex 19 will increase with Human. Fighter 8 was to get HP up, but it still seems short. So maybe trading out to Ranger 11 for the No-Dex-Req IPS would be a good idea. Thinking out loud here; human 6 rogue, 12 ranger, 2 fighter might be a better combo. Of course, a lot of the armor and weapon feats are lost in that exchange as well.

The real strength of this build is the synergy between the Mech's Enhancements with the DWS's specialized shots:
-Called Shot, Sniper Shot capable of +2 critical multiplier and threat range (I don't think this stacks) and bluffs the target(s)
-Merciful Shot capable of dealing well over 500 scalable damage on Sneak Attack.
-10 Sneak Attack Dice
-45 meter PBS & Backstab range
-Capable of roughly 50% doubleshot with action boost (not including items or external sources of doubleshot)
-Can plow through multiple targets
-Vorpal hits on 19-20 will knock down targets (that can be knocked down)
-Leg Shot used to slow down targets and keep the from getting to you too quickly

Wearing heavy armor is certainly an option, though evasion is lost and some skills are reduced.
Skills should focus on search, disable device, and open locks for decent trapping. Spot and jump are also recommended.

Grouping: Put an intim tank out front, grab aggro, and let the ballista bolts backstab the life out of the bad guys.
Soloing: Run away! Run Away! Oh, and jump a lot or find a perch.

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Code:
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Ballestaer 
Level 20 True Neutral Drow Male
(8 Fighter \ 6 Rogue \ 6 Ranger) 
Hit Points: 270
Spell Points: 50 
BAB: 18\18\23\28\28
Fortitude: 15
Reflex: 18
Will: 5

                  Starting          Feat/Enhancement
Abilities        Base Stats          Modified Stats
(28 Point)       (Level 1)             (Level 20)
Strength              8                     8
Dexterity            18                    19
Constitution         13                    15
Intelligence         18                    22
Wisdom                8                     8
Charisma             10                    10

                  Starting          Feat/Enhancement
                 Base Skills         Modified Skills
Skills           (Level 1)            (Level 20)
Balance               0                     4
Bluff                 0                     0
Concentration         0                     2
Diplomacy             0                     3
Disable Device        n/a                   n/a
Haggle                0                     0
Heal                  0                    -1
Hide                  0                     4
Intimidate            0                     0
Jump                  0                    -1
Listen                2                     3
Move Silently         0                     4
Open Lock             n/a                   n/a
Perform               n/a                   n/a
Repair                0                     8
Search                2                    10
Spellcraft            0                     6
Spot                  2                     3
Swim                  0                    -1
Tumble                n/a                   n/a
Use Magic Device      n/a                   n/a

Level 1 (Rogue)
Feat: (Selected) Point Blank Shot
Enhancement: Mechanic (Rog) - Arbalester (Rank 1)
Enhancement: Mechanic (Rog) - Sharpshooter (Rank 1)
Enhancement: Mechanic (Rog) - Awareness (Rank 1)


Level 2 (Rogue)
Enhancement: Mechanic (Rog) - Mechanics (Rank 1)
Enhancement: Harper Agent - Agent of Good (Rank 1)
Enhancement: Harper Agent - Strategic Combat I (Rank 1)


Level 3 (Rogue)
Feat: (Selected) Rapid Reload
Enhancement: Mechanic (Rog) - Sharpshooter (Rank 1)
Enhancement: Mechanic (Rog) - Skill Boost (Rank 1)
Enhancement: Mechanic (Rog) - Skill Boost (Rank 2)


Level 4 (Rogue)
Ability Raise: INT
Enhancement: Mechanic (Rog) - Skill Boost (Rank 3)
Enhancement: Mechanic (Rog) - Use Magical Device (Rank 1)
Enhancement: Mechanic (Rog) - Use Magical Device (Rank 2)
Enhancement: Mechanic (Rog) - Use Magical Device (Rank 3)


Level 5 (Rogue)
Enhancement: Mechanic (Rog) - Tanglefoot (Rank 1)
Enhancement: Mechanic (Rog) - Wand and Scroll Mastery (Rank 1)
Enhancement: Mechanic (Rog) - Sharpshooter (Rank 1)


Level 6 (Rogue)
Feat: (Selected) Insightful Reflexes
Enhancement: Mechanic (Rog) - Targeting Sights (Rank 1)
Enhancement: Mechanic (Rog) - Wand and Scroll Mastery (Rank 2)
Enhancement: Mechanic (Rog) - Wand and Scroll Mastery (Rank 3)


Level 7 (Ranger)
Feat: (Favored Enemy) Favored Enemy: Undead
Enhancement: Mechanic (Rog) - Sharpshooter (Rank 1)
Enhancement: Mechanic (Rog) - Fletching (Rank 1)
Enhancement: Mechanic (Rog) - Fletching (Rank 2)


Level 8 (Ranger)
Ability Raise: INT
Enhancement: Mechanic (Rog) - Intelligence (Rank 1)
Enhancement: Mechanic (Rog) - Fletching (Rank 3)
Enhancement: Mechanic (Rog) - Leg Shot (Rank 1)


Level 9 (Ranger)
Feat: (Selected) Weapon Focus: Ranged Weapons
Enhancement: Mechanic (Rog) - Mechanics (Rank 2)
Enhancement: Mechanic (Rog) - Awareness (Rank 2)
Enhancement: Mechanic (Rog) - Intelligence (Rank 1)


Level 10 (Ranger)
Enhancement: Deepwood Stalker (Rgr) - Far Shot (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Increased Empathy (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Increased Empathy (Rank 2)
Enhancement: Deepwood Stalker (Rgr) - Increased Empathy (Rank 3)


Level 11 (Ranger)
Feat: (Favored Enemy) Favored Enemy: Dragon
Enhancement: Deepwood Stalker (Rgr) - Sneak Attack (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Favored Protection (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Empathic Healing (Rank 1)


Level 12 (Ranger)
Ability Raise: DEX
Feat: (Selected) Improved Critical: Ranged Weapons
Enhancement: Mechanic (Rog) - Sniper (Rank 1)
Enhancement: Mechanic (Rog) - Mechanical Reloader (Rank 1)


Level 13 (Fighter)
Feat: (Fighter Bonus) Improved Precise Shot
Enhancement: Mechanic (Rog) - Sharpshooter (Rank 1)
Enhancement: Mechanic (Rog) - Rapid Fire (Rank 1)
Enhancement: Mechanic (Rog) - Rapid Fire (Rank 2)


Level 14 (Fighter)
Feat: (Fighter Bonus) Power Critical
Enhancement: Mechanic (Rog) - Rapid Fire (Rank 3)
Enhancement: Deepwood Stalker (Rgr) - Sniper Shot (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Empathic Healing (Rank 2)


Level 15 (Fighter)
Feat: (Selected) Heavy Armor Training
Enhancement: Deepwood Stalker (Rgr) - Thrill of the Hunt (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Thrill of the Hunt (Rank 2)
Enhancement: Deepwood Stalker (Rgr) - Aimed Shot (Rank 1)


Level 16 (Fighter)
Ability Raise: CON
Feat: (Fighter Bonus) Weapon Specialization: Ranged Weapons
Enhancement: Deepwood Stalker (Rgr) - Damage Boost (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Damage Boost (Rank 2)
Enhancement: Deepwood Stalker (Rgr) - Damage Boost (Rank 3)
Enhancement: Deepwood Stalker (Rgr) - Thrill of the Hunt (Rank 3)


Level 17 (Fighter)
Enhancement: Deepwood Stalker (Rgr) - Favored Hunter (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Killer (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Merciful Shot (Rank 1)


Level 18 (Fighter)
Feat: (Fighter Bonus) Heavy Armor Combatant
Feat: (Selected) Toughness
Enhancement: Stalwart Defender (Ftr) - Toughness (Rank 1)
Enhancement: Stalwart Defender (Ftr) - Durable Defense (Rank 1)
Enhancement: Stalwart Defender (Ftr) - Durable Defense (Rank 2)
Enhancement: Stalwart Defender (Ftr) - Durable Defense (Rank 3)


Level 19 (Fighter)
Enhancement: Stalwart Defender (Ftr) - Stalwart Defense (Rank 1)
Enhancement: Stalwart Defender (Ftr) - Stalwart Defensive Mastery (Rank 1)
Enhancement: Deepwood Stalker (Rgr) - Killer (Rank 2)
Enhancement: Deepwood Stalker (Rgr) - Killer (Rank 3)


Level 20 (Fighter)
Ability Raise: CON
Feat: (Fighter Bonus) Greater Weapon Focus: Ranged Weapons