Seath and his tail vs magic only build

Hi 
I'm currently running with my mage on NG: 
str 16, dex 11, int 37, I have all the combat sorceries that I can use.

The only thing that I use is Monlight Spear (got it to +4), Sorceries and Monligth Sword ... at least when I'll get it. 
(+some armour, elf ears and dragoncrest ring)

This is where the hard part comes, I simply cannot cut Seath tail. 

I stand by the crystal, let him destroy it while I run to stab the tail. I hit it like 2-3 times for 40 damage, when he starts his tail attack. 

Tried with magic: hit the tail with Soul Spear a couple of time and almost killed Seath, so I quit/load. Then I tried soul arrow it does almost no damage after several hits he got me with his tail. Still no tail trophy.

I've never had that problem with my other builds (usually a a heavier combat weapon). 

So my questions are:
- can you cut the tail with sorceries attacks? 
- is the tail cut after several hits or after a certain number of wounds dealt or maybe it's random?
- is there some trick to get that tail, f.e. when you stand here and here all atacks directed at seath will hit his tail

Try the slumbering dragoncrest ring. Seath is blind, this will give you extratime to maneuver behind him. Use the Clay to chip his tail, then proceed as you like best.

off topic but also on:

what is the best elemental or type of weapon to use against Seath? I am about to have a go at him on my current Pyro/strength/dex build

also, what is the best armour to use for the battle? anything with good curse resistance like Painted Gaurdian and will I need magic defence like the Stone Armour?

Everything including fire damage makes him go off like roasted chicken. Armor? His attacks can be easily avoided. Best protection is a good stack of soft humanity.

Painted armor set has good magic and poison resistance, but I use the antique dress set which has the best curse resistance.