Yup, start over. It's never too late to do an introduction properly, or so they say. (And this can work for almost anyone who's having problems with their kitties getting along, or so the feline behavioralist I got the instructions from says -- she even says cats that have known each other for 4 years without getting along can benefit by this procedure).
The most likely way to succeed is to separate them entirely. Try these steps (if you can follow them -- Skye and Blaze wouldn't follow them exactly when we introduced Sienna
)
First, select a room that will be the new kitty's room and ONLY that kitty's room for a short while. This will be for anywhere from 1 to 3 weeks. With Sienna, we did this for about 3 days. The kitties set the pace.
Leave NC (new cat) in the room as much as possible, while RC (resident cat(s)) is left *out* of that room. Don't allow them contact with each other except under the door. If you hear hissing coming from either RC or NC, don't open the door and let them mingle.
Keep NC in that room until RC no longer shows any hostile interest in the door. Playing under the door is okay. Hissing, hostile posturing, growling, excessive tail-swishing, ears laid back, intense focus on the door without leaving even to eat... those are all signs of aggression. Wait until those have all passed before allowing NC and RC to see each other again.
When you do go into NC's room or decide to let NC out for a little while, lock RC in another room for the time that NC is out. That continues to maintain the separation between them.
Once there's no more aggression between NC and RC when they can only smell each other, move on to step 2. This could take days, it could take a couple of weeks. Be patient.
For step 2, switch the kitties for a bit. Let NC run and play in the house for a few hours, while RC is in NC's room. Do this several times, still not allowing the cats to have contact with each other if possible. Try to observe how NC reacts in the house, and how RC reacts in NC's room. They'll be getting used to each other's scent, and starting to allow their scents to mingle and become a "colony scent" so to speak. Again, if there's hissing or posturing, back up and try again the next day. If there's not, move on to step 3.
Step 3 is to get a few baby gates or screen door or something that can be set up in the doorway of NC's room so that NC and RC can see each other but not get to each other. We did an inexpensive screen door and it worked great.
This time, you're going to leave NC in his room and let RC have the run of the house again until RC stops being angry at NC. Don't let them have physical contact with each other until RC stops hissing at/swiping at/growling at/being aggressive towards/etc NC.
Again, you can switch places when you want NC to have run of the house for a few hours.
You should keep the screen door/etc up until there is no more aggression between the kitties. Again, may take a few days, may take a few weeks, be patient. With us, it took 2 days of the screen door before Blaze stopped growling at Sienna through it.
Once they can get along with sight, then it's time to let them loose together for increasing periods of time. Don't start out with a full day right away. Give them a few minutes together first, then let them take a break. Then, when a few minutes no longer causes trouble, let them try an hour or two. Increase in greater time spans until you can leave them out the entire time someone is there to supervise.
We did have to do this first with Skye, then with Sienna, and both times it's worked. Hasn't been as fast as we wanted, but with time it did work. And in our cases, it was faster than I honestly expected -- but in our cases, all three kitties grew up with large litters in family households, so I think that greatly improved the introduction process.
According to the behavioralist (and those who have followed her instructions and posted about them on ivillage), this method works on the majority of cats. Not all, but most of them. I've only read of a few people who still didn't have luck with this method. I'd have to go back and see if I could find what she suggested when this method didn't work if you can't get it to work or if you're interested.
Also, when you're introducing them face to face, have a fishing-pole type toy and treats available, as well as something with which to make a loud noise. I found a fishing pole with a string and feather on the end of the string was the best solution for me. When the cats started making growling or hissing noises, I used it sorta like a whip -- not at the cats, NEVER, EVER at the cats, but to fling really hard at the ground about 5 feet from them. It makes a loud SNAP that breaks them up every time. (It's akin to slapping a rolled-up newspaper on the ground near a dog.) Then, as soon as they're separated, I use the toy as it's meant to be used and thus distract the more aggressive of the kitties. Turn the agressive attention from the other cat to the feather toy instead. If the cat won't turn attention to the feather toy, it probably means you need to back up a step in introductions again. (And yes, I had to do that, too)
Another thing I found was when they were in the same room, not attacking each other, but eying each other aggressively, I'd toss them each a treat (starting with the resident cat). That turns their attentions from "hostile" to "oh, food!" and teaches them to associate good things with each other. That, also, seemed to help with Blaze and Sienna (he really didn't want to accept her at first).
An accelerated run of this method has worked for us with introducing both Skye and Sienna, and with the friends who got Maribelle, the stray kitten we caught a few months ago.
Hopefully it'll help you, too!!
Also -- when they are interacting -- watch them. Vocalizing does not necessarily mean one is hurting the other. DON'T put your hand down to check, but if you can see, try to see if the claws are out and if the teeth are actually clamping down or if they're just "counting coup" (or whatever the spelling of the phrase is). Sienna screams up a storm if either Blaze or Skye do much to play with her, but they aren't hurting her in the slightest and as soon as I break them up (again with a loud noise, not by getting between them), she's the one to pounce and start the play-fighting again. Usually teeth and claws aren't involved. So try to observe and see if you can read body posture -- are they being noisy but playing, or are they being aggerssive? Hisses are almost always aggressive. Mrowls, mrrrs, and squawks may or may not be -- those need to be taken in with other body posture and actions in order to determine if it's aggression or play.
I hope all of this helps!! If you want to know where I got most of it, check out the book Think Like A Cat by Pam Johnson-Bennett. I've really learned a lot of how a cat thinks and behaves and why they behave that way from her book.
Lily cannot tolerate the new guy (we need to give this poor cat a NAME! 
) at ALL, She is very aggressive towards him. He spends most of his day under our china closet. Now this does not surprise me because he is very timid. (When we do hold him, he is sweet and purrs and purrs. ) But Lily goes around the house LOOKING for the little guy. And when she sees him, she hisses and takes swipes at him. 
