Ok, i was about slayer also because of leech and was thinking about getting Dark Pact also. My idea was about Cyclone coupled with Ethereal Knives (with the "throw them in a circle jewel)/Bladefall cause i wanted to keep it physical damage, but ince it's all about spells i can easily use a phys->lightning conversion and focus on getting to the top section of the tree then.
There are a couple nodes really useful around the duelist anyway. The huge hp cluster, and also the +2 melee range (which i read it's a great tool for CoC cyclone) plus eventual 3 nodes that boost resistances pretty hard. Also, great tip about power charges - i wanted to use a curse in some way either triggered or classic lighting orb/coh method. Again, i can change the triggered spell.
Thanks for the tips. Yes, the upper left was just a typo
EDIT: i meddled with the tree a little and went with
THIS.
I tried to get all the crit related nodes, leaving room for more hp (which i think i'll need); pure spell damage nodes are the only ones at witch starting point, which is reachable with some effort. I left away all weapon related nodes since they boost attack damage and depending on what i choose i can get some "+crit with weapon" (if they apply globally).
Strategy is leveling with the spell of choice (be it EK or else) and start using cyclone when i unlock it together with CoC. Prolly leaving cyclone the lowest level possible to have low mana cost and avoid too high stat requirements.