The Sylas build for Mid is Everfrost and First Strike. This LoL Sylas guide for Mid at Platinum+ on 13.23 includes runes, items, skill order, and counters.
Sylas Counters
52.2%
(+0.7%)
48.8%
(-0.7%)
8.7%
(+0.1%)
11.0%
(-4.5%)
791
(-31)