First off you need to find a random scatter item, a grinning skull (A small black skull is here, grinning at you.) they are vanish-death and decay in a few hours, so using them soon is always advised. This guide is more for soloing Tellanet since finding multiple skulls is hard to find, should you have more than one skull, the same tactics will work, she will just die a lot quicker.
First off you need to sort out your equipment, minimal armor is advised (since finding a second skull to loot should things go wrong is hard), so long as you achieve high 90's across head and torso you will be ok. Familiars from the orb of power (Mark of Balance) and Agrippa rings are an advantage if you have them.
If you have no timepiece for dispelling, having a dispelling weapon ready for the fight will help a lot, octarine quivers are the best since they dispel much faster, but for non rangers a heaven's wrath (expiring anyways so it's ok to lose) or a pike of retribution (cpk proof, but means no shield) will be ok. Be warned though she can be immune to any of these weapons.
A enervating weapon ready on hand for draining her spell points, spikebit, demented blade or wand of dark are good choices. As with the dispelling weapons she can be immune to these weapons too.
Obviously this depends on your classes but as a monk or ranger you should mix you around 10 fatigue mixes and around 60 high explosive mixes (she's yet to be fire immune when I have killed her, but you never know when that could change).
Check your memorized spells, redo them if they are close to fading, stock up on reagents if you are low, set your combat defense to dodge, take out your bronze bell if you have one or a staff of summoning (obtainable from necromancers in Catacombs of Shame), get some low level enervating weapons such as whisper blades (from assassins in Frost Giant's Keep) and grab a mount if you have riding, an Ithrix ward is recommended.
Once ready and fully spelled up, regenerate to max stats then hold and use the skull.
A few things to consider first, Tellanet and her mount both bite paralyse. She can cast bolt type spells doing up to 2000hp damage depending on hit locations so try to stay above 2000 hp until you drain her sp, she only seems to have around 1500sp. The ardor shortsword does mental-lash type damage, Ogres beware. She loads with a black flag that has random immunities and resistances each repop. The room it's self is no regen and CPK.
You will find your self in a non pk room in the Ethereal Void, it's no regen so don't be wasting sp here. Use your summoning item if you have one and make four air type elementals (use 'item name' air). Set formation structure to shield and give all elementals a low level enervating weapon and order them to equip it then haste the elementals. Once ready Tellanet is located down, going up will take you out of the Tellanet area and into the normal Ethereal Void area and unable to get back to her. Once you are in Tellanet's room fleeing up will have the same effect of you not been able to get back to her, fleeing south, east and west from her room all seem to have a increased chance of teleporting you into the normal Ethereal Void, fleeing north from her is the best option and least likely to teleport you out in my experience.
Once down, proceed to try dispelling her haste, if you get paralysed change your defense to auto or shield block in case she bolts. String for her bolting is Tellanet begins muttering an incantation... if your are not paralysed and you see that spell string flee north, wait at least 3 seconds for the spell to finish casting before going back in the room.
When she is dispelled immediately change to your enervating weapon, throw a fatigue mix and cast curse, faerie fire, weaken, then try to either plague or poison on her (she can be immune to plague and poison or very resistant to them, you need to land at least one of them, so spam till you land). Once spelled up proceed to use all your HE up, and try to stay above 1200hp. Try not to bash since the room is no regen and using all your sp and st up will leave you dead. When your sp is depleted flee north a second or two before the next tic and recover (any healing items or cabochon are a help as the longer you spend out of her room the more chance there is of been teleported away from her area).
From here on its just a matter of casting cure light every so often and throwing a new fatigue mix roughly every 4.5 to 5 minutes. Eventually she will drop to screams in agony and try to flee. Letting her flee is a good idea, as the rooms around her are not no regen and will make the final phase a lot easier in my experience. Having said that if she flees up you will be in the normal Ethereal Void, this is a real pain some times as she can be immune to web and chasing a paralysing mob around in the Ethereal Void is not fun! Should she flee any other direction but up, room shield if you can, then follow her, try your best to web her, but as previously stated she can some times be immune.
Once you get her down to barely clinging to life, its time to be very careful again. Order the elementals to flee so you can monitor damage more effectively, as she gets to her last few hp you need to over cast heal onto yourself if you are approaching a tic. Should you kill her a few seconds before a tic you risk been attacked by the mount, this is very bad as the mount is extremely buff and paralysing. It will disappear after a few tics, but that's more than enough time for it to kill you. So time your tic correctly and try to kill her immediately after one, any items she carries will drop to the ground so grab everything and get out of the room quickly. One final warning the ardor shortsword is trapped, if you have very low hp when she dies it will kill you when you pick it up.
Now find your way back to the no regen room with the up, go up and scan for Mordred, go to him grab a vial if there is one, drink it to get out, or puzzle if you have one.
Item 'a yellow block of a magic cube' is type toy, alignment -1000, made of unknown, has keywords 'black magic cube yellow '. This item weighs 0 stones and 1 pebbles, and is valued at 28,800 gp. This level 241 item has the attributes: identified A yellow block of a magic cube is in excellent condition. This item has a default lifespan of 90 days and will expire on Thu Nov 18 23:28:12 2010. *This item may be repaired 5 times and renewed 4 times. This part of a magic cube looks familiar, although it's not exactly clear why. It has funny little grooves on its uncolored face, which seems like it ought to provide some clues, but alas. It seems to wiggle like a bean, as though there were a worm inside, but that seems unlikely. It's from gaining more, similar pieces, and discovering what this the adjacent pieces link to. Item 'a locked journal' is type book, alignment 0, made of paper, has keywords 'bound journal steel relic book artificing'. This item weighs 0 stones and 15 pebbles, and is valued at 260 gp. This level 10 item has the attributes: identified A locked journal is in excellent condition. *This item may be repaired 5 times. This journal cover is bound steel, complete with a miniature, rusted lock. The lock isn't old enough to be broken easily, just enough to make its key impossible to insert. The key is probably lost in time anyway, and it's magically protected, resistant to both magic and lockpicking. Whatever is inside is interesting enough to protect, and protect well, which is unfortunate. Because it is, indeed, protected well. The only chink in this journal's cover is a few loose pages, but only one of them has strayed enough where you might be able to pull it out a bit more than is already poking out. If you tried hard, you could probably read at least a few of the page's details. Item 'a shortsword named 'Ardor'' is type weapon, with keywords 'shortsword sword ardor passion emotion vitriol' and is equipped wielded as a weapon. A shortsword named 'Ardor' is in excellent condition. Weapon type is sword, does 180 points of mental-lash damage for 2 stamina, accuracy 13. With your knowledge of 'sword', this weapon can cause 231 damage against 0 ar. Special weapon attributes: vorpal *This item may be repaired 12 times. It is alive, burning brightly with passion and emotion-filled energy. It is alive, it is thinking, it is. It occasionally emanates a mental burst, frightening everyone in the vicinity. When it wriggles, it lashes out at you physically, attempting to slice your arms. If Ardor may be harnessed, it will be a very powerful weapon indeed. This item is beyond your power to identify further.