I will not vote since I have a guide for her myself, but 21/7/2 is pretty random and a silly attempt to cover up the fact that you bring 0 defensive runes (not to mention you're denying yourself of Runic Affinity , and yes Syndra can go oom even with Grail).

You may counteract the rune comment with what you said in your guide, but that still leaves you open to good all-ins (like levels 2-4 Red Elixir Kha'Zix), and you further contradict yourself by stating Liandri and Rylai's are core when they provide poor ap (not to mention Liandri's is pretty lame on her save for few occasions, whereas Deathfire Grasp is a lot better). The rest of the guide has way too many text walls which colors alone will not improve.

Both Spell Weaving and Blade Weaving are terrible on Syndra. In lane phase, it does not do enough damage, and in teamfights you will rarely ever auto attack someone.

Hey, it's a great guide, you explain things well but I suggest you use more BBcoding. You can check jhoijhoi's bbcoding guide, it is really easy to understand and things look cooler. There's a lot of colors and I like it.

For the items, I am not a big Syndra player, infact I dont even own her but I HAVE player her and I think there are some items that you should Reconsider. Example Deathfire Grasp. Deathfire Grasp is such a nice item for syndra because she can deal a lot of damage in short time, but against some tanky champions your combo's will not kill them, only make them low and make them back. Another example is Zhonya's Hourglass. If you are against an enemy such as Zed or Talon you might want to rush this item instead of buying it last.

Also, your runes look weird, I am not saying they are not viable but Greater Seal of Scaling Ability Power are not really worth if you think about it. Greater Seal of Armor gives you 13 armor at level one, unlike Greater Seal of Scaling Ability Power that gives a total of 16 ap at level 18. You might want to consider your runes or at least make a second choice against champions such as Zed (AD).