I'm asking this because I use poisons a lot, mostly I make them my own because I've mastered alchemy but I still haven't started the main quest, if any of the monsters in the Oblivion gate have poison resistance I'll probably be screwed because I use it so much on my bows. If any of them do have poison resistances can anybody give me some advice on how to dispatch the enemies in there? I'm a lvl 15 thief/assassin type character with really bad magic skills and 80 in marksman.

All atronachs are immune to poison. Fire and frost atronachs are weak to frost and fire damage, respectively. Storm atronachs have no weaknesses.

My character is a mystic archer, so she is all about using magic to enhance her bow killing.

For poison immunes, she precedes the shot with one of a variety of custom spells that stop her foes and busts their poison immunity.
The essential effects she uses on these spells are: Weakness to poison 100%x5sec + Drain speed 100x1sec. The weakness element obviously busts the immunity, and the drain speed element resets their AI making them go stupid for a second so they forget they were just hit by a spell as long as she is hidden when she casts it. They then stand unaware while she quickly shoots her poisoned sneak bow shot.

Here is a http://i668.photobucket.com/albums/vv43/Acadian6/Buffy%20I/ScreenShot94.jpgfor example (ghosts are immune to poison). Buffy is patiently waiting for the poison to kill him. She can one shot kill a storm atronach this way as well with a stout poison.

That type of spell works wonderfully on foes who are not immune to poison as well. For immunes, you get normal poison effect. For non-immunes you get double the poison effect.
Beyond those basic elements, she has variations of the same type spell that add a long term silence or a long term soul trap effect - depends on the foe.

These are rather elaborate measures to take, but Buffy won't touch a melee weapon, so she has to be good at what she does.

That's actually an awesome idea I never thought of that, I guess I'll have to do the Mage's Guild Recommendations now, where do I get the weakness to poison spell and Drain speed spell, can I buy it at the discount staffs guy in the market district in IC? And how much magic points would it take to cast this spell, I'm not really a big magic character.

Weakness to poison you can find in IC, drain skill should most likely be in Skingrad.
You will however need an alter of spell making to be as cunning and subtle as Buffy ( and she sounds very cunning ).
Perform the mage quests to get into the tower, buy the mages tower dlc and invest in it early, or find a mod that adds one to a house somewhere.

Weakness to poison spells can be purchased from Ungarian (Bravil), Calindil (IC) or Sulinus Vassinus (Skingrad).

Drain speed is available for enchanting if you have any drain attribute effect. There is no such spell to buy. It is available as a reward for completing the Orerry DLC or via a small handful of other ways - detailed here: http://www.uesp.net/wiki/Oblivion:Drain_Attribute

The spells of this nature Buffy uses are kind of pricey magicka-wise, but that goes down sharply as destruction skill goes up.

Don't forget to get good at making poisons too, so the effects of those weakness spells are optimized with powerful poison.