I'll fight both and play them against each other until both are dead.
That's just how I do it.
Enjoy.

Congrats for the most creative response yet. You are the winner, and I hope this situation never happens to you!

Giant snake, with a giant mongoose.
"She hasn't even read the books" - Elliot_Alderson

I would smother the snake in Greek Fire.

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I mean.I'm sure it has been done in movies and such plenty of times haha. Hell in Sharknado a guy cut himself out of a shark!

I've seen it done, and can't exactly remember where. I'm guessing we saw the same source.
You don't have to be faster than the shark

POSER!

Giant snake with a dagger. A giant snake is either going to try to constrict you, or swallow you. Either way, you can cut it open with a sharp knife.
You don't have to be faster than the shark

A giant spider because even Samwise could defeat a giant spider. With a sword of course.
My name is Darth Jar Jar. Master of the Drunken Force technique. All of you will be my slaves.

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Snake.
Greek Fire.
Wouldst thou like the taste of butter? A pretty dress? Wouldst thou like to live deliciously?

Spider: fight it with a Bongo FLAMETHROWER

You could always borrow bongo's.
Wait..What?

Also, what is worse than a giant snake or spider coming after you??? A FLAMING GIANT SNAKE OR SPIDER COMING AFTER YOU!
LOL!
As for your question, snakes aren't the slightest bit scary to me, no matter how big you make it all they amount to is a mouth and a tail and are limited in numerous ways.
Spiders on the other hand are only non-scary because they are so small. Even a tarantula or camel spider squishes nicely under the heel of a combat boot. However a
giant
spider who could not be dispatched so easily would be quite alarming, especially if it retained qualities like being able to climb anywhere and if its webs strength were scalable as well. Being trapped and helpless while a monstrous arachnid ate you alive would be amongst the worst ways anyone could go.
So on second thought, I'll take my chances with the flamethrower!
'Honi soit qui mal y pense'

Yeahhhh but Greekfire didn't cause massive explosions like wildfire. Greekfire was used mostly for naval warfare (if I remember correctly).
Also, what is worse than a giant snake or spider coming after you??? A FLAMING GIANT SNAKE OR SPIDER COMING AFTER YOU!

Still, there is Greek Fire, Wildfire is based on:
https://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Greekfire-madridskylitzes1.jpg/1920px-Greekfire-madridskylitzes1.jpg

"Art is magic delivered from the lie of being truth"

You jabroni! Clearly I didn't intend for this to be GoT related or I would have said OT! I'm talking realistic.with..an unrealistic creature.

I thought the Alchemists' Guild was developing a Wildfire based device for Aerys II, before the Maesters took over under Robert.

"Art is magic delivered from the lie of being truth"

Am I missing something? They didn't have flamethrowers back then did they?!

Spider, flamethrower if no dragon handy.

"Art is magic delivered from the lie of being truth"

I'd kill the spider with a war hammer flush to its abdomen, cut off its sticky legs with a shortsword on the way to the mother lode. The snake would receive a pike thrust to the throat. Make it a very long pike.
As of today, I have read
31
pages of Game of Thrones

you could jab at it with a giant snake
After all is said and done, a lot more will have been said than done.

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