In order of chances of this happening highest to lowest.

1.The 'natural' way.
2. Be Teela Brown who is the sixth generation of a line of ancestors all born because - in each case - both parents won a Birthright Lottery and probably end up like Anna.
3. Beam me up Scoty
4. Befriend a pred willing to eat you and then through you back up.

If it was me i would try my luck cutting my way out with a titianum knife, machete, or katanna that has a dimond edge that is sharpend to a point 1 micron wide.
If it was mac he would also due the same or just force his way up ( trust me mac is freakishly strong)

Ah thanks but I'll leave last order as is...

I know I sure as hell am not a Teela Brown my luck sucks horribly

Actually, I wouldn't be so certain of the sharp weapon technique.

A human stomach has five layers, at least two of which are muscle, plus mucus protection from the gastric contents used for digestion, and atop that is generally very elastic. Considering the size of the stomach in relation to the food (our human survivalist, in this case) and the nature of the physical qualities that protect the body from the harmful material used in digestion, I'd say comparatively speaking you might not be able to cut your way out.

Especially when we start to examine the weird cases where people swallow sharp objects for no apparently good reason. Open safety pins, scissors, knifes, etc. The stomach is an elastic organ and it compresses to churn - there is no reason that an object of comparative size to the weapon a human would be wielding in the predator's stomach (or something that no human could ever hope to wield) not piercing the stomach wall would be able to pierce the stomach wall in our scenario as presented.

Granted, our human has the benefit of localized strategic application with his blade, and that will give us a force multiplier over the argument of people swallowing dangerously sharp items without issue. I'm still skeptical though. That mucus membrane is going to make finding purchase with either foot, hand, or blade difficult at best. The idea of humans walking around inside a stomach always struck me as 'meh' given the internal linings and churning action.

Still, the human stomach has a wall of 2.80 ± 0.12 to 4.23 ± 0.03 mm varying thicknesses so if we assume crude 1:1 ratios, then a 100ft giantess stomach would have 44.667 mm ~ 71 mm of thickness which is certainly not thick enough to stop even a short blade lending credence to possibility of being able to pierce through it.

I would propose a shaped charge or concussive weapon to be a better bet for you than a blade, provided you can do something to minimize the damage to yourself from the explosion, the shock of the body's response, the chance of ricochets, and the mess you're going to find yourself quickly upon in the body cavity. Maybe if you were aiming to take them out at the same time as you, but I'd say without outside help you're probably still going to die. Gravity is a rather cruel mistress.

Of course, this entire thread is missing the biggest point of all - not being eaten in the first place. I generally chalk things up to once you're past the esophagus you're not coming out except as waste in these scenarios. Some things suggested in the earlier posts reek of embellishment.

I'd say, bring an accordion with you, and play that for them inside the stomach until they eventually force you out and flee. Works even better if you don't even know how to play the accordion.

If you've got ice magic or some insanely cold stuff, like liquid nitrogen, you could freeze your way out.

Yea, like said just before, I've always wondered what if someone tries to use fire or ice (magic) inside a pred's stomach.
Freezing all the stomach wall AND the acid should normally hurt the pred and keep you safe for a while.
If the ice is kept for a long time, stomach's membrane may be damaged, and eventually start to drop bodies temperature (not counting that without the acide, the pred can't digest you or anything).
Same goes with fire constantly used on stomach wall, tough there you have quite less chances cause you may end up "toasting" yourself xD (unless we consider that being a fire mage, you have some resistance to heat) and you have no way to escape the acid.

In the case of ice, I think it would be preferable for the pred to let you go and try her chance on another human. ANd for the fire, depends of the wall resistance, but I think cells can't handle that much heat.

Also, keep in mind that sustaining a fire requires oxygen, so it may increase the rate at which you suffocate inside the stomach.

Most likely, trying to start a fire is going to wind up with you roasting yourself in short order.

Ice would theoretically work much better, especially since it could readily damage the lining. Freezing moisture could readily do some nasty damage to the stomach that, even though it may be healed by the soil's nature, would prevent consumption for some time. Anywhere there's moisture, though, using Ice Magic has room for being evil when applied internally.

Yea, I wasnt really convinced for the fire, ice is defenitly a better choice

Edit: though now that I think about it, would ice magic still work on a magic resistant stomach?
After all, the magic doesn't affect "directly" the stomach, but decreases the temperature, and magic resistance doesn't mean resistance to changes in temperature, doesn't it?.