Well i managed to get in and get out alive! At lvl 2 i followed Foster's advice (cheers !) and drew them out in the daylight. I didn't notice them taking too much damage from the sun though...
I engaged the first undead in a swordfight not taking much dmg (spared my bottles

) and then got in and out with the next 2.But i realised Bethesda (spelled well i hope) hasn't managed to program the game any better than Jowood in Gothic 2. I jumped on a high enough rock outside the caves and 99% of the times the enemy couldn't follow! I then took out the one without armour and weapons with 2 flares and then used a lot of flares on the orcish character. When both dead i went back in, found the summoning undead char. far away from the last undead warrior so he went down with 2 flares too (strange enough he summoned no creatures but decided to use his bow :S). Against ranged attacks hiding behind pillars and coming out to release some magic proved very efficient.Then i went to find the last enemy (second and last tough one for me) and he was stuck in a tunnel trying to come close to me but always colliding with two pillars there!He died with a ton of flares from a far distance...But i guess so much for A.I. ! Then activated god mode to carry every item found back to a merchant...call me cheater but it spares time on going back and forth...maybe i will install the horsebags mod so it is more realistic. Just afraid to make the game buggy that way...
Thanks for the suggestions everyone!