Honestly, its extremely easy to Juke Xerath's ult and all his skills altogether. Just head one direction for one second then turn the other way. Do that 3 times, and his ults wasted with you just walking/running around. Riot designed his ult to be very dodgeable and that they did do. ALL his skillshots give warning signs when they're about to be cast making it very easy to dodge. Out of all the champions to nerf, Xerath is the one champion that in my opinion, is completely fine. All skills are dodgeable and if you can't dodge them, you're not playing against Xerath right.
As for your xerath's, it is also easy to ks with his ult BUT I'm sure I read somewhere that riot intended xerath to be able to do this. He's the type of champ that needs help. He has a ton of cc but no dmg.....a good balance in my opinion. idk what your xeraths have been doing but if they haven't been slowing or stunning and ksing baron and dragon, thats the teams fault, not his.
Basically, all skills are avoidable and if you can't dodge them, then you're not playing against him right.