TieredItem

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

ZenScript
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import crafttweaker.api.item.tiered.TieredItem;

TieredItem extends MCItemDefinition. That means all methods available in MCItemDefinition are also available in TieredItem

ZenScript
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// TieredItem.getTier() as IItemTier

myTieredItem.getTier();