the general gameplay is this:
you pick a weak enemy lane, and you push them off their tower, then you take that tower or the dragon. if you took dragon you have to repeat until you take tower.
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when you area ahead:
now you use some opportunity to put up wards in that side of their jungle, and you either (a) know that their jungler is there so you go bully the other side of the map, push them off tower and take tower, or (b) gangbang enemy inside enemy jungle (with wards and sweep the area first with red trinket!), push them off that side's tower, and take that.
rince and repeat a couple of times, until only base towers left. Then you either group up to bully them away from base towers to take those, or use careful split push pressure (harder in your mmr) to do same effect. You withdraw into baron and bait a baron fight.
baron and dragons done at opportunities, or if enemy anti-siege too strong and you can't poke them out of position.
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if you are behind, you have several good strategies. ASSUMING you're not a bunch of spineless impotent scrubs who only play assassins and need somebody to carry your ass 24/7. When behind you need to be proactive and take risks and understand the risk/reward curve is AGAINST YOU.
you can group up and blitz through laners that havn't grouped up yet. Then you do the same process as above only you stay grouped and you do it really really fast and keep the momentum going (if you are behind the enemy team being unbalanced and not calculated play keeps you from losing). if you can keep the momentum going you can keep enemy in losing state despite gold lead.
you can setup superior vision in your own jungle and catch the enemies as they try to ward or make plays. This is not easy because they are trying to do the same to you, so a skill matchup between the supports actually.
you can set up cheeky deep wards in enemy jungle where they think they are safe and gangbang one of them when they are poaching camps. This is good way to also tilt that person.
you can use a pick or two especially against jugnler to take baron, then group and push with the baron.
If you have a really annoying hard to catch duelist (jax, trynd, singed, maybe Riven, Camille), ask him to split push. these guys can't be first into the action even if they are mega fed. they can join in vie teleport or flank AFTER the fight is developed and mop up. if he can annoy the other team enough, that's your comeback.
if you are behind you have to be super smart and careful but you also have to be aggressive and not let opponents choke you out of resources. you are not just trying to regain power (your gold VS opposing team gold) but also the control over who gets to make which play. unless your late game is amazing, "be safe and farm up" is not necessarily effective.
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when you upset the game and are again in lead position, see above when you are ahead.
Notice that behind game-play is much more open ended. This is because the end result of behind play is much broader (get back into game gold wise) than ahead play (progress towers down), and objectives are taken more as an opportunity than as a matter of gameplay.
All of this you have to figure out how to put into playing Xerath. He's got his own strengths and preferred strategies that connect to this broad view of things.
3
then if you get ahead purchase 
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when you can and just poke and farm, if you don't get ahead dont bother buying 
then 
then 
. Xerath isn't an average champion and should have a relatively high kill participation just because of his access to long range poke which allows you to garner more assists/kill which can lead to stacks on 
