Killing a tier I is mostly dependent upon 1 thing: "how open are the wizard's options?"

If you go with "anything except being Pun-Pun" then you run into wizards doing things like casting [epic] spells before level 21, running around with NI spells, NI spell DC's, arbitrary caster levels, arbitrary numbers of actions per round, having epic mythals on their person granting obscene powers, etc.

If you start limiting options, then you have to start limiting options for the "killer" since the tier system is based on allowance of similar levels of optimization.

The very best you can hope for is a stalemate, really.