With the 1.10 synergy effects, it is very important how you spend your skill points. Ideally, a CL/FO built should have 18-20 in Lightning, 20 in CL and 20 in LM (Lightning Mastery).
Realistically, your character will not reach a higher level than late 80's, so let's say you have 15-20 skill points left to allocate. Max out CL first, put the rest in LM. FO will be your main skill, you use CL + your merc only agaist cold immunes.
The Merc is actually more helpful than one may think, and for a sorcess the act 2 Desert warrior is the best choice, since you can take advantage of his aura. You can choose from Defensive, Offensive and Combat, and you should go for a defensive one. If you plan to play the areas, a Merc with the Holy Freeze aura is the best. This one you must rent at Nightmare, he is the Defensive. If you plan to do mostly boss runs, a merc with the Defience aura might suit you better. That one you rent either in Normal or Hell, and he is also Defensive. If your merc is going to be of any use, it's very important that he is the same level as you, and that he has good equipment. My suggestion is that you go back to Act 2 Nightmare and get a HF merc, then you give him an armour with very high defense, a polearm that does lots of damage, and make sure he has lots of life leach. The merc can use ethereal items without wearing them out, so you can take advantage of the +50% bonus ethereal items have

Give him Tal Rasha's mask, Crown of thieves or some other helmet with life leach, socket his weapon with an Amn rune for life leach or a Shael rune for attack rate. My Meteorb Sorc's Merc has a Bonehew (unique polearm) with an Amn and a Shael, and Ta's mask.
If you rent a HF merc in Nightmare, he will be many levels lower than you. This means you must level him up, and this is best done where you found him. Merces are very sensitive to XP penalty, so to gain good XP, he must kill things himself and the monsters must be approximately the same level as he. So play through Nightmare again, from Act 2, and level up your merc. Put on a lot of Magic Find gear (topazed helmet, armour) in order to make it less tedious

If you don't want to level up your merc, wait until Act 2 Hell and rent a Defiance Merc, he will then be the same level as you.
Good luck!