With only the DMG and PH to draw from, you've not a lot of options, but here's how you can
Hit Often & Harder
Below's a build that does what you want it to do.
Prof. Littleman, Male Halfling Bbn1/Ftr2/Rgr2/Rog13/Blackguard 2
Character Class BAB Feats Sneak Attack
1st Rogue 1 +0 Two-weapon Fighting (PH 102) +1d6
2nd Rogue 2 +1 +1d6
3rd Rogue 3 +2 Weapon Finesse (PH 102) +2d6
4th Rogue 4 +3 +2d6
5th Rogue 5 +3 +3d6
6th Rogue 6 +4 Quick Draw (PH 98) +3d6
7th Rogue 7 +5 +4d6
8th Rogue 8 +6 +4d6
9th Rogue 9 +6 Improved Two-weapon Fighting (PH 96) +5d6
10th Rogue 10 +7 +5d6
11th Rogue 11 +8 +6d6
12th Rogue 12 +9 [choice] +6d6
13th Rogue 13 +9 +7d6
14th Ranger 1 +10 Track (PH 101) +7d6
15th Ranger 2 +11 Rapid Shot (PH 99), Greater +7d6
Two-weapon Fighting (PH 95)
16th Barbarian 1 +12 +7d6
17th Fighter 1 +13 Power Attack (PH 98) +7d6
18th Fighter 2 +14 Cleave (PH 92), Improved Sunder (PH 96) +7d6
19th Blackguard 1 +15 +7d6
20th Blackguard 2 +16 +7d6
Prof. Littleman flanks with his allies and stabs the crap out of folks until he can afford a ring of blinking (DMG 230) (27,000 gp; 0 lbs.). After he's bought the ring--typically at character level 9, but maybe 8 if he's content throwing rocks for a level--his weapon of choice becomes flasks of either acid (PH 128), alchemist's fire (PH 128) (20 gp; 1 lb.), or both.
With flasks and the ring, Prof. Littleman stays at range. One of the effects of the spell blink [trans] (PH 206) is to deny foes their Dexterity bonuses to Armor Class versus Prof. Litteman's attacks. As he's employing splash weapons, Prof. Littleman makes ranged touch attacks versus his foes. Splash weapons can inflict precision damage in Dungeons and Dragons 3.5.1
Thus at character level 9, while using the ring of blinking, Prof. Littleman takes the full-attack action to make 4 ranged touch attacks that each inflict 6d6 points of fire or acid damage. At character level 15, he takes the full-attack action to make 7 ranged touch attacks that each inflict 8d6 points of fire or acid damage. This is as good as Prof. Littleman's going to get until character level 20 when his number of attacks increases to 8.
The real issue the Professor has is encumbrance. His light load is 24 lbs., yet each attack he can make weighs 1 lb., and there's very little he can do about that. If 3 rounds worth of flasks are insufficient, it's time for Prof. Littleman to employ the Use Magic Device skill on that tucked away scroll of teleport [conj] (PH 293) (DMG 240) (1,125 gp; 0 lbs.) or to find cover or concealment and make Hide and Move Silently skill checks while he while quietly retrieves additional flasks from his Heward's handy haversack (DMG 259) (2,000 gp; 5 lbs.).
Note 1: The levels in blackguard (DMG 181-3) are to eek out those final +2 points of base attack bonus while A) getting class features, and B) avoiding an XP penalty for multiclassing (PH 60)--if the DM enforces that rule--after the Professor reaches epic levels.
Note 2: Taking 2 more levels of blackguard for +1d6 sneak attack is an option by replacing the level of barbarian and the second level of fighter, but doing so requires taking the feat Power Attack at character level 12 or taking the rogue special ability feat and picking the feat Power Attack.
Beyond the ring of blinking and items that improve the Professor's ability scores, Armor Class, and saving throws, the Professor will want a melee weapon or two. A wand of flame blade [evoc] (PH 231) (2nd-level spell at caster level 3) (90 gp/charge) creates an effect that's incompatible with the feat Weapon Finesse but nonetheless allows the Professor to make melee touch attacks for damage therefore inflicting sneak attack damage when the foe's susceptible. Later, the weapon special ability brilliant energy (DMG 224) (+4 bonus)--considered overpriced by many--permits the Professor's attacks to ignore armor bonuses and shield bonuses to Armor Class, which, while not as good as a touch attack, allows him to employ the feat Weapon Finesse.
Given the Professor's Strength, Small size, and the dearth of magic items in the DMG that grant an effect like the 4th-level Clr spell freedom of movement [abjur] (PH 233) (e.g. the ring of freedom of movement (DMG 232) (40,000 gp; 0 lbs.), for 1 hour/day if a 10 is rolled on the SRD's epic minor artifact Olidammara's dice (ELH 152) (artifact; 0 lbs.)), initially a scroll then a wand of freedom of movement is a good investment. The blackguard and ranger facilitate this purchase by having the spell freedom of movement on their spell lists.
Dealing with Creatures Immune to Precision Damage
Constructs, oozes, undead, plants, creatures with the incorporeal subtype, and elementals (COUPIEs) are the creatures types in the core rules that are immune to the rogue's sneak attack damage. There's little the core rogue can do to mitigate that immunity to precision damage. However, unlike the fighter--who lacks Use Magic Device as a class skill and probably dumped Charisma, too--the Professor should be able to employ the Use Magic Device skill to cast a spell from a scroll or wand that will solve his COUPIE problem. A scroll of limited wish [univ] (PH 248) (7th-level spell at caster level 13) (DMG 240) (3,775 gp; 0 lbs.) is a good last-ditch response to a host of ugly questions.
- Splash weapons can't deal precision damage in Pathfinder.