The Janna build for Support is Shurelya's Battlesong and Summon Aery. This LoL Janna guide for Support at Platinum+ on 13.24 includes runes, items, skill order, and counters.
Janna Counters
52.6%
(-0.9%)
40.8%
(+0.6%)
3.4%
(+0.1%)
0.6%
(-0.1%)
185
(-2)