Hey, glad you liked it! It sure is underrated, Tibbers with that slow = perma slow. :D

For me, Xerath is one of the 2 most Hardest match-up for Annie, the other being Syndra. All you have to do is play defensive. What I would suggest is that you should use Movement Speed Quints to help dodge all his abilities since all are skill shots. Rushing boots and an Abyssal Scepter is also nice. Farm under tower and ask for a gank especially if he's aggressive since Xerath is like you, very immobile. Another option would be for you to roam to the side lanes and try to get a kill or an assist.

Well facing Xerath is like facing Brand, I forgot to add him to the threats-list. What you do against longer ranged champions is that you try to avoid a fight until you reach 6, and when you got Tibbers you should have a stun available and flash-ulti the person instantly. You don't need to have a stun but it's way easier if Tibbers enrages(especially if you the Rylai's Scepter). So unless you got your ultimate up or your jungler is up for a gank, try to avoid fights. That's what I would have done. Late game you should have no trouble to nuke him or any other mage down. Also if you see that he try to use his ultimate on you, try to turn back and stun him. I personally never had issues vs Xerath or Brand after around Level 6. But until then just farm farm farm and you should have no issues!