Hi,I have been palying Malphite as support for a few weeks and,well,you can see it for yourselfs ;)
https://www.youtube.com/watch?v=Viw55dmXI3I
https://www.youtube.com/watch?v=VGjeGt2Qb6g
https://www.youtube.com/watch?v=renxOyUeOjo

Attack damage runes becaouse with his w and e it is easy to harras any adc.
Magic resist just in cases that MID decides to gank you and it will most likely and up
dead (enemy mid)
Movement speed for your q,it will help you in chasing enemies or in rare cases escaping enemies

They are support masteries for 10% cooldown reduction.

I think these are standard for malphite JUST REMEMBER,DONT POKE ADC BEFORE LVL 3-5.Lot of mana used by q and not so much damage.Harras them with w and standard auto attack,it will be enough

Flash for everything-Escaping,chasing and if you are team chasing you cna use flash and than your ultimate or even better be behind wall so they cant see you and than flash + ultimate for good teamfight started.
Dont ever focus on enemy tanks!

From silver V to silver 2 with 75% win rate with Malphite and this build.Hope it grows even more.

Team work is needed becaouse carry with malphite is a little dificult.In team fights try to ult more than 2-3 enemies but if you ult 1 enemy make sure he is hard to kill.If he is down teamfight is yours.

PROS

-Passive Granite Shield makes enemy loose mana more faster
-Has great engage with Unstoppable Force
-Can kite well with Seismic Shard
-Can blow up squishy targets with combo
-Peels well for carries
-High base damages

CONS

Slow auto attack
Reliant on Unstoppable Force to be really useful in teamfights
Reliant on cooldowns
Mana problems if you dont know how to use it