Probably have an accident.

Well the armor and shield will be pretty much useless and the sword is a piercing weapon, so I run underneath it and stab it in the body until it stops moving.

==>Abscond.

befriend it
 
and yeah  
i can make that work

Summon something nastier to take care of it. It’s what we do in Melnipony.

Pee on it.
 
Then die horribly.

Beat it with little effort because games these days are easy as fuck even on “insane” modern difficulty.
 
Fucking casuals and their fucking hydras!

Die horribly.
 
Because realism.

Only one chance - you can’t go for the heads, since while you’re attacking one, the others will eat you.
 
So the only option left is to:  
fight your way into its butthole.
 
Once you crawl inside its intestines, you can take your time slicing it apart from the inside while it slowly dies of blood loss. Meanwhile, you’re in no danger yourself.
 
Battle over.

The hydra ran into the old stone arena, knowing that a fresh  
pony meal would wait there.  
While entering the arena the hydra accidently brushed her big  
tail against the stone walls, causing them and the great entrance  
portal, to collapse behind it.
 
Twilight Sparkle observed the event anxiously, she was too  
exhausted to just teleport out of the arena and the edge  
was too high for her to jump on. And now the entrance  
wasn’t an option any more.  
She didn’t miss the irony of the situation, being trapped  
against a hydra with no proper escape route, again.
 
The cold autumn air tugged at her mage tunic and her  
little emergency sword hung from her waist.  
Vanguard and the others surely needed her now, so  
she had to make this as quick as possible.
 
Just as Twilight made up a scheme to defeat the beast,  
the hydra charged at her.  
It leaned forward, the eyes of the four heads glinted  
with anticipation. Its tail stretched outward to hold  
balance, its two big legs stomping the ground.
 
The beast was now only a few meters away from Twilight,  
a smug winning grin already plastered over its face.  
Suddenly Twilight dashed forward, she accelerated herself  
with the little magic she had left, directly between the  
hydras legs.  
At the other side of it flung herself around, all four hooves  
skidding on the flat stone ground while she leaned  
forward to keep her balance.
 
In an attempt to snatch the charging unicorn, one of the  
hydras head hit unceremoniously the stone and was now raging  
with anger.
 
The moment Twilights momentum was exhausted she jumped on the  
tail of the hydra, galloping her way up the still forward  
leaning beast.
 
The raging head was the first to spot its reluctant meal  
on its back. It flung itself sideways in an arc to catch  
the unicorn from the side.
 
Twilight saw the raging head homing in from her right side,  
she grabbed the short sword with her telekinesis and threw  
it at the head, but the head dodged and she only inflicted  
a rather small cut on its cheek.  
Fortunately this distraction was enough to give her the time  
she needed. Twilight blindfolded the nearest head with  
a small spell and bucked its neck, causing the head to tumble  
down a bit. She sprinted up its neck and from the top  
of the head she took a leap of faith to the edge of the arena.
 
And then she saw it, the raging head was on direct collision  
course with her. The big cream coloured teeth dripping with  
saliva. Twilight tilted her upper body towards the incoming  
head and mustered every drop of magic she had left.
 
A picture of Pyre Valor flashed up in her memory,  
her stern face looking at with a expression of  
“If you fuck up, die and break your promise.  
I will personally torture you in hell”.
 
And then everything went down in slow motion.  
Twilights upper body was tilted strangely toward  
the raging head, beads of sweat on her forehead.  
Her horn crackled with magic, but no fireball formed  
before her.  
And then the sharp teeth were directly around her, she  
smelled the stinking breath of the beast and could see  
its throat.
 
But then the fireball formed and Twilight instantly shot  
it. It only travelled a few centimetres before hitting  
the mouth area of the beast and thrusting the head away,  
smoking.
 
Twilight unceremoniously hit the hard stone of the arena  
terrace. She groggily got to her hooves again, her left  
side aching badly, to look for the hydra.  
But to her relief the it didn’t make any attempt to  
hunt her again.  
She limped down the terrace and saw the silhouette of  
a pony.
 
“Vanguard I…” she croaked and collapsed.
 
 
 
Sorry for the mistakes and sometimes dull phrasing.  
It was just a little something written in hurry :P

Comment on OP’s spelling and grammar long enough to summon Robo Grammar Hitler.

Entangle all the heads with my tunic
 
Slit all the throats with the Rusty Shield, giving all the heads tetanus

Jack off surely this will help.

They didn’t seem very smart in the show. Run under it’s legs.
 
Barring that. If I can do so in one piece, get myself swallowed and hack my way out.

Throw sword at hydra.
 
Run.

cut a head off burn it shut and repeat

I give it meat.  
Pet it heads.  
Ride it and escape the arena.

Quit the game and play Zork instead, where I will be summarily eaten by a Grue because I forgot to bring a light.

Maybe run under it like Twilight did and try to disembowel it.

I die.

OPPA GANGNAM STY- “gets nom’d”

Die. Most likely painfully.

>look

Spread rumors among the multiple heads. Causing disharmony among the beast and eventually it offs itself as all it’s heads gnaw at the other for not getting them that birthday present they’ve always wanted or other various reasons.