Grab a load of blocks and speed build a ramp way up to it and punch it in its stupid monkey face. If it has friends, they'll run in fear from your manliness and lack of fear since you just punched a flying guy that shoots fireballs in the face.
Or you could just hide and wait for it to come to you and stick it with infinity arrows while you hide in a cobblestone or likewise bunker. But that's not nearly as awesome. Not even close.
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Science is good when it teaches you how to turn a million ungrateful foreigners into glass.
I've gotten pretty good at shooting them with arrows from a long range. You just have to adjust for the arrow's trajectory. Also, you can hit/shoot fireballs back at them, although I wouldn't shoot at them unless I had a ton of arrows to spare. I have a ton of feathers and flint in the normal world, so I take a couple stacks of arrows with me, and I can run on that for a long time. If I run low, I pretty much hide and dig around the ghasts. My nether base is all cobble, and where I have other blocks, I put cobble under/around them, so that I don't have a massive pit if my grass blocks get destroyed (Yes, they're hacked in. The Nether is so bland, otherwise. Deal with it.). By the way, does anyone know if Notch made it so ghasts can't destroy lightstone/whateverit'scalled? I have a doorway lined with the stuff, and it hasn't been blown up yet, but I don't want to test it out for fear of losing it, since it's such a PITA to get.