The Milio build for Support is Shurelya's Battlesong and Summon Aery. This LoL Milio guide for Support at Platinum+ on 13.23 includes runes, items, skill order, and counters.
Milio Counters
49.6%
(-2.0%)
40.2%
(-2.3%)
2.7%
(-3.5%)
1.5%
(-0.8%)
139
(-53)