With health regen from Quas orbs and tangos, you can stand inside Alchemist's Acid Spray and deny creeps! Forge spirits are primarily used to deny runes. It's secondary use is to last hit. In some cases, you need to deny creep A with Invoker and deny creep B with forge spirit. Forge spirit's third use is to scout/block camps from stacking. Of course you can stack your own camps, but it is more important to stop Alchemist from farming.

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