Well now, if you want to kill the barbarian no matter the cost (and I mean that quite literally), I think poison has it's merits. Especially so, if, as the OP conceded, our rogue gets a surprise round.
Warning: Ideally I assume the rogue gets the drop on the barbarian, but he could always just have his poison already applied to his (efficient-quivered)arrows. I don't know of a rule that the poison would denaturate.
I'll take 12th lvl, 20 point-buy as a basis for this example.
STR
A "typical" B will max Str, or at least go for 16+2 racial, give str another +2 for lvls and we arrive at an unmodified str of 20.
DEX
Let's slap on 15 and get +1 from lvl = 16
CON
Leaves 16 for Con
INT
Dump for 7
WIS
11 to not ruin the Will-save
CHA
Dump for 7
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At lvl 12 you could either get a belt of +2 for 2 ability scores
or for one +4. To make the B be more resistant vs poison let's assume the +2/+2 (as I'd wager, a B wouldn't normally get nothing for Str)
Rage is already greater at lvl 12 so another +6/+6 to Str and Con
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Raging: (nonraging)
Str 28 (22)
Dex 16
Con 24 (18)
Int 7
Wis 10
Cha 7
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Base Fort-save 8
Cloak of Resistance +4
Con +7
Trait +1
= Fort 20 raging (17 nonraging)
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AC
10
+4 Mithril Breastplate 10
Dex +3
Amulet of Natural Armor +2
Ring of Deflection +2
= AC 25 raging (27 nonraging)
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Rogue goes Full-Dex, a bit Con to live longer and Cha cause rogues are dashing ;) Poisoner Archetype as not to poison himself by accident.
Str 8
Dex 18 +2 racial +2 lvl-up +4 belt
Con 13 +1 lvl-up
Int 10
Wis 10
Cha 12
Str 8
Dex 26
Con 14
Int 10
Wis 10
Cha 12
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BAB 9/4
Dex +8
Longbow +3
weapon focus +1
Pointblank Shot +1
Rapid Shot -2
Manyshot
Boots of Speed
=Full Attack: 20/20/20/20/15
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Take Swift Poisoning (so your preparation doesn't take ages), Deadly Cocktail (2 doses of poison on one arrow, DC+2 frequeny +50%)
Every dose of poison ups the DC by +2, buy or craft 12 (24) doses of Deathblade poison (Deathblade, injury 20, 1/rd. for 6 rds., 1d3 Con, 2 saves, 1,800 gp a piece)
So:
1 arrow hit: DC 22
2 arrow hit: DC 26
3 arrow hit: DC 30
4 arrow hit: DC 34
5 arrow hit: DC 38
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Nonraging B
We hit the B with 7-up (picture that) so that's a 65% chance for our "good" attacks, 4*65%= 2,6
"bad" attack hits 40% of the time= 0,4
let's assume 2 hits or 3 hits.
2hits save DC: 26, 3hits save DC: 30
Fort-save 17
B makes 2hit-save with 55% chance of success
B makes 3hit-save with 35% chance of success
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Raging B
We hit the B with 5-up so that's a 75% chance for our "good" attacks, 4*75%= 3
"bad" attack hits 50% of the time= 0,5
let's assume between 3 or 4 hits.
3hits save DC: 30, 4hits save DC: 34
Fort-save 20
B makes 3hit-save with 50% chance of success
B makes 4hit-save with 30% chance of success
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If the Barbarian Counterattack
If he blows his save he'll get 1d3 Con damage which will make it harder to make the save on future rounds. 2hits will make this 9 possible rds, 3hits will make it 12 rds and 4hits will make it 15 rds.
If we won Initiative or got the drop on him we'll have another round to apply more poison on a weakened B if we survive his counterattack (I'm too lazy to do that now, maybe use the Rogue-Talents "Resiliency" or less ideally "Another Day" ).
Conclusion: If we blow our money on poison we get a pretty good chance of killing the Barbarian (even if he may kill us before he bites it).
Yours cordially,
an Asurendra