Procarbine
Forever Platinum
I don't really understand how Dark Seer is supposed to work (in standard solo offlane position). Do you intentionally push the creep equilibrium to their tower with ion shell, so that their tower messes up their last hits? Meanwhile, you get your gold/xp from casting ion shell on their jungle creeps, while hanging around in lane for xp/occasional ion shell lane creep kills, surge-running away as soon as they go for you?
Yeah that's what you do. Get a soul ring and you try to run them out of regen and either kill them or stop them from farming comfortably at all. Ion shell has the added bonus of punishing pulling pretty well, if the carry has to tank a wave with ion shell they're going to get worn down much, much faster. You can also start cutting the creep wave with level 2 ion shell if you get into a comfortable position in the lane.
Also Anti-Mage because Dark Seer kind of needs his mana.
Edit: Getting a value point in Mana Shield is also pretty good against Ion.
I think ds does decently against am. Early on he'll be able to have an ion shell up if am blinks on him and can trade hits easily. Later you'll have soul ring and ring of regen to keep the ion shells going.