This guide obviously is more for fun since you'll be playing on the Proving Grounds. I round that it works quite well, actually. Unlike on most other maps, Soraka doesn't fall out of the competition on the Proving Grounds. Her ability to sustain, keep allies alive, and harass the enemy team bit by bit is crucial in a map wothout the luxury of healing at the base. You will mold Soraka into the ultimate source of life on your team, and a decent source of pain for your enemies. But beware, though Soraka is powerful and helpful, she cannot escape her fatal squishiness. Don't let the enmeies catch you without a fight, and don't let them kill you without a loss on their side as well. This is how you prove that Soraka is the best damn support you could ask for.

Runes and masteries are your own business to deal with. Build whatever you think is necessary . Honestly, they won't make a world of differece in this case. Since you all start at level three and are constantly teamfighting, you won't get much downtime for regeneration to do a lot of good, and the flat enhancement buffs will quickly fall off or immediately be countered by opposing items since the starting gold and passive gold gain is increased on the Proving Grounds. Also, it's only available for custom matches, so I wouldn't recommend trying to construct a rune or mastery page simply for it unless you plan to play it exclusively. However, if I had to suggest what types of runes and masteries you should get, I'd say go primarily for mana regeneration and health regeneration, secondarily for cooldown reduction, and lastly for a bit of health, armor, and magic resist to stay a little sturdier if you really think you'll need it. However, most of your stats will come from your build, so if anything, try building all of your runes and masteries towards one specific thing, such as ability power, health regen, mana regen, armor, magic resist, health, etc. Doing this will essentially play the role of standing in for one item. That way, it's like you get one free item from the start, plus whatever you can buy. This also helps you ignore that stat for the time being. That's just how I would do it, but like I said, you should just use whatever pre-existing rune and mastery pages you already have.

Pretty standard stuff as far as skill sequence goes. Get 1 point in your three main abilities at the start since you get all three poitns immediately. This is so you can feed health to allies, feed mana to allies, silence enemies, and harass enemies as well. Remember that you can hit enemies from a greater distance with infuse than you can with starcall, so don't be a hero under the turret with starcall, but don't waste all of your infuses on the enemy because your team canalways use the mana to spam more.
After that, put a point in your ult every chance you get, max out your heal, then mak out infuse, and lastly max starcall. Remember, your main job is to keep your allies full on health and mana. Reducing enemy magic resist and doing small bits of damage with starcall is secondary to that, although the increasing stacks on starcall's debuff will help your ap carries out quite a bit in fights, even in the early stages of the game.

You're all about sustsin, which means you want to give your team all of the health and mana you can possibly give them. So, naturally you'll want to get HEAL and CLARITY. You will most likely find yourself low on mana a time or two, as well as many of your teammates at once sometimes. This is where clarity comes in great handy. Instead of trying to wait for a bunch of infuses for you team, and trying to wait out the mana regen on yourself, simply use clarity for a quick boost. Normally it's not a great spell, but it works wonders on the Proving Grounds. Also, heal is very common for this map anyway. Both of these spells enhance your main goal as well, keep allied health and mana up.

Your build is desinged to do what both your skills and summoner spells already do, keep allied health and mana up. Get BOOTS OF LUCIDITY right off the bat to keep you from getting haraased too much early game as well as to get some nice cooldown right away. Also, you should be able to complete a kindlegem along with it for even more cooldown and a little bit of health. Finish up your SOUL SHROUD to reduce everyone's cooldowns and up their mana regen while giving yourself a nice load of health in the mix. This will aid the mana issues easily and give your team more time to attack since they will be able to spam faster. Then build ZEKE'S HERALD so that every time your team auto attacks they get some health back with that lifesteal, prolonging sustain. Then get WILL OF THE ANCIENTS so that every spell your team casts heals them as well, further prolonging sustain. Next is LOCKET OF THE IRON SOLARI which passively gives an aura of health regen for your entire team, which furthers sustain even more. Also, its active places a weak but useful shield on everyone within range, giving your team an edge in teamfights by redicing a decent amount of damage. Lastly, if you end up getting this far without a victory, finish it up with AEGIS OF THE LEGION to buff your teams armor and magic resist to make them even harder to kill. Also, the little attack damage bonus will couple nicely with Zeke's.

No wards.
No pots.
No gold per 10.
No AP focus.

Build for your whole team, play safe, play smart, and you will easily steamroll your enemies by wearing them down bit by bit while you stay alive.

PROS
-Great sustain for all
-Wears down enemy team over time
-Mostly eliminates the problem of not having spawn pad healing and relieves need for retreating to pick up health on the sidelines behind your team

CONS
-Does not do immense damage alone or fast, needs to wear down enemies over time
-Not very durable, needs to stay in the back when injured or focussed, must play safe
-Relies on team to pick up most of the kills and take most of the damage

This build will create an immense amount of sustain for your team. Lifesteal, spellvamp, health regen, heal summoner spell, and your two healing spells all kep your team's health bars up. Infuse, clarity, and soul shroud buff your team's mana pools. The locket's shield plus the armor and magic resist of aegis make your allies ery durable. Plus, all of these items include other stats to help boost your teammates' capabilities as well as your own. If you want to sustain all day on the PRoving Grounds, this is the way to go. No one else gives you that much power over your enemies. Now go out there and prove to the world that you're the best damn support there ever was.