The Syndra build for Mid is Shadowflame, Stormsurge, and Luden's Companion. This LoL Syndra guide for Mid at Platinum+ on 14.3 includes runes, items, skill order, and counters.
Syndra Counters
51.6%
(-0.2%)
51.5%
(+0.4%)
10.5%
(+0.2%)
9.0%
(-1.2%)
840
(+20)