FSC Build.

Which, for simplicities sake would be the balanced build. If you ask Pandora, Deadbones, or the other high level PvP/TW players, a high con FSC is wanted. Which unless you heavily micromanage your stats, a Light Armor build will do you no good nor will the offensive builds.
Use the Balanced Build, and slot your armor for +HP, this will give you the survivability you need to heal as fast as you can in TW/FBs/HHs/TTs.

If you wanted even higher con, you would need to sacrifice your next tier weapon, and it would gimp your healing somewhat.

FSC is not a stat build but more a Skill build.

This mean that what matters most is the skills you chose. OFC if you go support then healings and buff hace max priority without leaving Plume Shot and Great Cyclone (mainly your only attack skills during a lot, a lot of lvls).
Buffs? Maybe you wont need them all at all (specially talking about your char) but others chars will need them. So you have to lvl up them all.
Celestial guardian is a special case. Cost a lot to lvl. If you use charm you actually dont need it. If you dont, well, it can save you a lot of time and in fbs you can almost do it without pots (suppossing others party members dont go kamikaze always).And when meditating you will gain mp back very fast.

For skills read guides, they all said what to take.

For being useful in TW, you just need to put Vit into your build. 6 mag 3 vit 1 str is the most FSC oriented build. More hp, lowest damage in cleric build. 7 2 1 is a bit more offensive, 8 1 1 almost pure mag, and 9 1 the ultimate mag build and very few base hp (expect to have less then 1900 base hp at lvl 73, without hp shards and vit equip, ofc).

So... just put vit and you will have your FSC. If you put vit I personally will recommend to take all +magic +magic attack equip you can find, you will also need to use the ultimate weapon avaible for your level. This is because your damage wont be good at all and it will be a bit hard to take mobs down.

What Ive personally experienced is this "oh, Im vit cleric, have a lot of hp, but take me a lot to kill mobs, so those Hp are there to actually get hit bcs I kill slower" then I re stated "oh, Im pure mag cleric, I dont have many Hp, but I kill mobs faster"... so it almost the same speaking in the PVE area. More Hp=more time to kill Less Hp= less time to kill.

Those extra Hp will be great with barbs buffs. It will maybe be just a bit more time consuming to lvl and kill mobs. Also you will need always to be updated with equip, if you slow down in that area you will notice it.

Hope this help from a guy who has been both vit and mag clerics.

Wield Thunder is pretty worthless for grinding, just as initial attack, even so, you better throw a cyclone then a few plume shots. The damage cyclone does comparing to time to cast and mana wasted, you could cast 2 plume shots or 1 cyclone+1 plume shot and would use less mana.
It IS a cool looking spell and does a nice damage, but costs almost 3 times more than its useful, its extremely useful on pvp situations though, cant plume heavy armor users, and you only have a few metal spells that dont consume chi.

As for razor, once you have maxed your necessary spells, or if you are battle cleric, max it sooner than any other, even thunder. It does around 1k more damage than plume shot maxed and has a nice AoE radius, being extremelly nice to mob train and on PvP (mass killing robe users). Remember, even if it is only 2k damage, you just did 2k damage on about 10 targets, total of 20k+ damage in a single shot.

Siren kiss will do about same damage as cyclone, that on lv6, ,v7 and higher it will actually do more damage (using a lv70 weapon, cyclone gets 100% extra that siren kiss doesnt). It also has a freezing debuff that lasts 3 seconds, which can be useful on both pvp and pve. At lv7, it can freeze 8/10 on any mob. Still working on maxing it to see it it gets near 100% (which the description had the real %).

I cant talk about thunderball since i left it at lv1, but you can be certain that 100% attack then 4k over time is a little trashy, considering you can simply throw a plume/cyclone once in a while and get same benefit (will just generate a little more threat for being burst damage). But i will endup maxing it after lv80.

Well, up to lv66, only attack spell i used was plume shot, all others were kept on lv1, just then i maxed cyclone, then razor at lv70 and all others after razor. Tempest is still lv1, but you can be certain that it will still do 2k more damage on pvp than a lv5 wield thunder (4k vs 2k on barbs).