Here’s my translation

Alan

Sá var mikill vexti. 
That-one (He) was tall in stature

Hann hafði jarnstaf í hendi ok var geir Óðins markaðr á framan verðum, ok syndist honum sem eitr sindraði ýr oddinum. 
He had an iron-staff (járn-stafr) in hand and (the) spear (presumably MnI spelling of geirr drops an r) of Óðin was marked foremost (framan-verðr, Z) on (it), and it seemed (sýndist) to him like poison sparkled (spluttered, sprayed?) out-of (typo, variant of úr) the-point.

Þat var Skjöldr, bróðir Hlégerðar. 
That was Skjöldr, brother of Hlégerðr.

Hann gengr at skíðgardinum ok lykr upp grindinni. 
He walks to the-wooden-paling-fence and opens spar-constructed-gate.

Flóka trippan hleypr þegar út. 
The-filly-foal-with-matted-coat (feminine form of flókatrippi) runs at-once out.

Ok öll hrossin eptir henni. 
And all the-horses after her.

Hesturinn fylgir henni. 
The-stallion accompanies her.

Hon hefir uppi ýmsa endana ok stefnir þangat sem blautastar váru mýrarnar, ok gefr henni þat líf at hrossin komust þangat eigi.
She has now-this-now-that end (of her body?) up (presumably describing her galloping action when going at full tilt?) and heads thither where wettest (softest) were the-moors (bogs), and that, that the-horses do not make-their-way thither, gives life to her (I can only interpret this as that her life depends on the other horses not being able to go to the wettest part of the moor?).

Skjöldr gengr til hellisins. 
Skjöldr walks to the-cave.

Hörðr sprettr þar upp fyrir honum. 
Hörðr springs up there in-front-of him.

Hann hafði gaddakylfu í hendi. 
He had (his) spiked-club in hand.

Hann sló til Skjaldar en Skjöldr skaust undan en kylfan kom í jörðina ok sökk it digrasta. 
He struck at Skjöldr but (and) Skjöldr moves-quickly (skjótast) away but (and) the-club came into the-earth and sank the deepest (ie as far into the earth as it could possibly go, ie completely buried)

Skjöldr sló til hans með jarnstafnum ok kom á kryppuna ok brotnaði í sundur geirinn. 
Skjöldr struck at him with the-iron-staff and (it) came on to the hump and the spear was-broken a-sunder.

En stöngin var föst í kryppunni á honum, ok kipti hann honum á lopt. 
But (And) the staff was fast (ie firmly) in his hump, and he pulled (catapulted) him (Skjöldr) a-loft (into the air).

Hordur spyrndi fótunum við bergit svá fast at Skyldi varð laus stöngin. 
Hörðr pressed with the-feet against the-rock so hard that the-staff (nominative) became loose for Skjöldr (ie Skjöldr was forced to let go of the staff)

Þá opnaðist hellirinn ok kom út þurs mikill, ok var þríhöfðaðr. 
Then the-cave opened and a great ogre-giant came out, and was three-headed.

Hörðr laust hann með kylfunni ok fell hann um þverar dyrnar ok var þegar dauðr. 
Hörðr smote him (it) with the-club and he (it) fell right across the-doorway and was at-once dead.

Þá losnaði stöngin ok náði Skjöldr henni. 
Then the-staff became-free and Skjöldr got-hold-of it (the staff, stöng is feminine).

Sló hann þá til Harðar í öðru sinni. 
He struck then at Hörðr for a second time.

Hörðr stökk undan ok inn yfir inn dauða. 
Hörðr sprang away and inside (the cave by going) over the dead (one) (ie the ogre-giant).

En stöngin sökk í þursinn er hon kom á hann, ok var föst við þursinn, en hendr Skjaldar við stöngina ok varð honum þá mikit fyrir at reiða hana.
But the-staff sank into the-ogre-giant when it (the staff, feminine) came onto it (the ogre-giant), and (the staff) was fast against (ie securely affixed to) the-ogre-giant, but (and) (the) hand of Skjöldr (was securely affixed) to the staff and (it) became for him then a great hindrance (verða fyrir e-m) to wield (brandish, reiða, Z6) it (the staff) (ie he could brandish the staff only with great difficulty).

Tröllin sóttu nú at Herði ok var þat mikil mannhætta. 
The-trolls now attacked (sought after) Hörðr and that was a great danger-of (to his)-life.

Skjöldr vildi nú inn í hellinn ok var honum óhægt um ferðirnar þvíat þursinn loddi við hann. 
Skjöldr wanted now (to go) inside into the-cave, and (it) was difficult for him concerning the-journey because the-ogre-giant stuck against him.

Ok fór hann mjök lágt í dyrunum. 
And he went very lowly (adverbial) into the-doorway (slithering on his belly under the ogre-giant?).

Hörðr opaði at honum ok setti stjölinn á nasir honum ok stukku úr honum allar tenurnar. 
Hörðr went-back to him and set the (his)-rump onto his nose, and all the-teeth sprang out of him (Skjöldr).

Hrokk hann þá út aptr ok vogaði hann þá eigi inn í hellinn þaðan í frá.
He recoiled (hrökk, hrökkva) then back out and he dared then not (to go) inside into the-cave from that time (see þaðan, Z3)

Hörðr barðist tvö dægr við tröllin. 
Hörðr fought for two half-days (24 hours) or two full days (48 hours) (doegr, Z1 or Z2, how do you tell?) against the-trolls.

Hafði hann þá drepit þau öll. 
He had then (by that time) killed them all.

Bar hann sik þá at at hlafa út. 
He bore himself then to (it) (set his mind to) to get? (can´t find hlafa anywhere?) out

Skjöldr stóð fyrir hellis dyrunum. 
Skjöldr stood in-front of the-doorway of (the) cave.