how to not feed with immobile mages midlane?

are they only situational picks? meaning if enemy laner has mobility/is an assassin, just forget about it? i'm talking the likes of xerath, vel'koz, etc i tried vel'koz for the first time yesterday and my biggest issue was how was i supposed to survive ganks, or assassins allins? barrier, E knockup and hope for the best? i'm new with those kind of champions so any advice would be appreciated
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