Mustrum_Ridcully
Legend
You don`t need 18s in specific stats - at least not before the use of spells and equipment 
A skill that is sometimes overlocked: Use Magic Device. It gives you access to spells, and it is one of the reasons why I recommend against multiclassing with spellcasters - you make the skill useless for many items.
(Oh, and before you multiclass with a spellcaster, check if you can`t spare a feat for "Leadership" and get a caster as companion!
)
If you can`t afford a Ring of Blinking, check a Wand of Blinking!(this is considered the ultimate killer tactic of getting sneak attack on every attack - at least, if you happen to be in the right dimension
)
Or even use Scrolls of Blink.
Sneaking during combat is difficult, but not impossible, especially at high levels using appropriate equipment (Cloak of Elvenkind/Robe of Blending and so on)
Bluff is useful to get Sneak Attacks at least every other round, combined with some good feats, it can really help you. (Combat Reflexes -> Expert Tactician).
It is possible to use ranged weapons effectively after the 1st regular round using invisibility or blink. (both from allies, wands, scrolls or other items, whatever you can get)
Speaking of Ranged Weapon, consider using Greater Magic Weapon from your allied spellcasters. (Though 50 arrows aìn`t much on a full day)
Teamwork is an important part on your way to success (though it is not always neccessary, some specific rogue concepts can work independendly, as our Swirfneblin Rogue in our high level campaign proved)
You and the fighter have to set up where you can flank your opponents and where the opponent won`t concentrate on fighting you.
(And if the only way to get to this is taking a Weapon Profiency in a Reach Weapon, do it. Better losing a feat than losing your life!)
Many opponents fight the first opponent they reach, and if this is the fighter, that`s great. So consider opening the combat with a few ranged shots (possibly with sneak attack), than let the fighter close into melee range with the target. Wait until the opponent attacks him, and than get into a flanking position.
(If the opponent figures out that you are the real threat - which is not neccesary true - he might switch towards you - but we are speaking of the 2nd or 3rd round of combat here, and chances are high that your opponent is near death anyway...)
If you have trouble with undead and other "immune to criticials and sneak attack", consider using Holy weapons (make your alignment a strength!) or lawful weapons (devils are underused, demons, vampires and drow are prefered opponents in several campaigns). Maybe even a undead bane weapon. (speaking of bane, the Scarab of Golembane is really cheap, isn`t it?)
If you can`t or do not want to use alignment based weapons, energy is the second option (maybe your DM allows even combos like Flaming and Frost).
(A side note here: Often I hear people recommend keen and improved critical for rogues - I don`t know why - most rogues do not gain much of it, because they don`t have high damage bonus that are multiplied with the weapon. Instead it makes them even more relying on targets that are subjects to critical)
If you can`t rely on Greater Magic Weapons, a high "plus" only can do the job - if gives you better hit chances, something a rogue can need.
The Short Sword of Subtility can be your weapon of choice, too. It gives a +4 bonus to attack and damage rolls when sneak attacking.
Mustrum Ridcully

A skill that is sometimes overlocked: Use Magic Device. It gives you access to spells, and it is one of the reasons why I recommend against multiclassing with spellcasters - you make the skill useless for many items.

(Oh, and before you multiclass with a spellcaster, check if you can`t spare a feat for "Leadership" and get a caster as companion!

If you can`t afford a Ring of Blinking, check a Wand of Blinking!(this is considered the ultimate killer tactic of getting sneak attack on every attack - at least, if you happen to be in the right dimension

Or even use Scrolls of Blink.
Sneaking during combat is difficult, but not impossible, especially at high levels using appropriate equipment (Cloak of Elvenkind/Robe of Blending and so on)
Bluff is useful to get Sneak Attacks at least every other round, combined with some good feats, it can really help you. (Combat Reflexes -> Expert Tactician).
It is possible to use ranged weapons effectively after the 1st regular round using invisibility or blink. (both from allies, wands, scrolls or other items, whatever you can get)
Speaking of Ranged Weapon, consider using Greater Magic Weapon from your allied spellcasters. (Though 50 arrows aìn`t much on a full day)
Teamwork is an important part on your way to success (though it is not always neccessary, some specific rogue concepts can work independendly, as our Swirfneblin Rogue in our high level campaign proved)
You and the fighter have to set up where you can flank your opponents and where the opponent won`t concentrate on fighting you.
(And if the only way to get to this is taking a Weapon Profiency in a Reach Weapon, do it. Better losing a feat than losing your life!)
Many opponents fight the first opponent they reach, and if this is the fighter, that`s great. So consider opening the combat with a few ranged shots (possibly with sneak attack), than let the fighter close into melee range with the target. Wait until the opponent attacks him, and than get into a flanking position.
(If the opponent figures out that you are the real threat - which is not neccesary true - he might switch towards you - but we are speaking of the 2nd or 3rd round of combat here, and chances are high that your opponent is near death anyway...)
If you have trouble with undead and other "immune to criticials and sneak attack", consider using Holy weapons (make your alignment a strength!) or lawful weapons (devils are underused, demons, vampires and drow are prefered opponents in several campaigns). Maybe even a undead bane weapon. (speaking of bane, the Scarab of Golembane is really cheap, isn`t it?)
If you can`t or do not want to use alignment based weapons, energy is the second option (maybe your DM allows even combos like Flaming and Frost).
(A side note here: Often I hear people recommend keen and improved critical for rogues - I don`t know why - most rogues do not gain much of it, because they don`t have high damage bonus that are multiplied with the weapon. Instead it makes them even more relying on targets that are subjects to critical)
If you can`t rely on Greater Magic Weapons, a high "plus" only can do the job - if gives you better hit chances, something a rogue can need.
The Short Sword of Subtility can be your weapon of choice, too. It gives a +4 bonus to attack and damage rolls when sneak attacking.
Mustrum Ridcully