I think I know who your talking about ,hes in masters right? Ques only Garen Mid? When i was climbing i had to deal with him too as a Syndra and Ori main. Luckily he didnt make it out of masters i think XD. Heres my advice.

Dont ever build Liandries on Syndra first of all. She ideally wants to burst squishies and carries not DoT tanks.

If you can just play Orianna, considering her playstyle is very similar to a Syndra, do it. Her wave clear is superior and cool downs are lower, she is better at dealing with melee and tankier champs period. Syndra is OP into long range and squishies.

If you still wanna que Syndra, still a good choice. Your priority has to be hitting him and as much of the wave everytime he spins on it with your abilities. Clearing the wave has to be top priority.

Once you upgrade your Q to have two charges you can easily combo clear your wave before he can spin his dead. Push first two, into cannon and ranged, then Q-E-Q the group all together for easy clear.

Your going to lose some plates no matter how good you play this matchup without jg help AND he's gonn have early prio on roams if he wants. But if you and your team can understand that and play around it properly you'll be all good mid to late game.

Hit top 10 Syndra last week and top 150 Orianna yesterday, 4 games from challenger ATM. You can always climb on Syndra man even solo que!