As agreed, I'm sending you the sentences I rephrased myself.

I'm looking forward to your prompt reply (as I need them for tomorrow). Thank you a lot

1) The breath of the dragon flared up from the rock and the ground boomed (re-echoed?)
Beowulf, concealed (“overshadowed”) by the mound raised his towering shield. Then he shook out (or drew?) his huge, ancestral, sharp-edged sword. The sword is described as an ancient relict (“heirloom”). Each felt terror at the other and wished the other dead.
2) The serpent slithered forward (“flowed forward”), coiled itself together and spat fire.
I still can paraphrase this line:
3) “The temper of the twisted tangle-thing was fired to close now in battle”. (does it mean that the serpent started to spit fire or that it lost its temper and spat out fire?).
4) Beowulf summoned the strength to swing his sword so hard that it snapped against the dragon's head. As it broke, it stroke the snake less strongly than required.
5) The snake became furious and started to spit death-fire.
5)Thristing for revenge ("seething with warspite"), the dragon charged a third time, seizing Beowulf by the neck with his poisonous fangs.
6) Beowulf is described as an intransigent but weakened (“sorely-straitened”) Lord of friends, a great chieftain and a brave warrior-King.
7) He embodies the courtly (?) ideal of the warrior: he is corageous, generous and strenuous. He is endowed with supernatural powers, too.
8) Actually, he can spend days under water, kill winged and fire-breathing monsters, which live underwater as well as underground.
9)The epic poem recollects historical events as well as (or together with?)supernatural and mythological tales

1. not "overshadowed" -- comma after "mound" -- "drew" not "shook out" -- relic (<~~watch the spelling) not "heirloom"

2. not "flowed forward" (snakes slither!) -- delete "itself together" -- "coiled up" is fine

3. for "temper" see verb definition 3 here: http://www.answers.com/topic/temper -- it means that the sword was made for close battle; it was a very well made sword.

4. use "struck" rather than "stroke"

5. "deadly fire" would be better than "death-fire" -- watch spelling ~~> Thirsting

6. "intransigent" is fine, but "weakened"???

7. watch spelling ~~> "courageous" -- I'd use "forceful" or "dynamic" rather than "strenuous"

8. Delete the comma after "water" and insert an "and" there -- without punctuation.

9. "as well as" is fine -- comma after "events"

1) To rephrase the first sentence: "The dragon's breath erupted from the rock, causing the ground to resonate." To get this rephrased sentence, I took the original sentence and replaced some words with their synonyms while maintaining the same meaning.

2) The second sentence can be rephrased as: "The serpent glided forward, coiled itself tightly, and spewed fire." Again, I used synonyms to rephrase the sentence while keeping the same meaning.

3) To clarify the meaning of the sentence: "The serpent's temper intensified, ready to engage in battle." In this case, it seems that the serpent's temper became flared up and ready for combat.

4) The fourth sentence can be rephrased as: "Beowulf summoned his strength to swing his sword with great force, causing it to break upon impact with the dragon's head. Unfortunately, the broken sword didn't strike the snake as strongly as intended." Here, I rephrased the sentence to make the meaning clearer while using synonyms to express the same ideas.

5) The fifth sentence can be rephrased as: "Enraged, the serpent unleashed its fiery breath." I rephrased the sentence to express the serpent's anger and its action of spitting fire.

6) To rephrase the sixth sentence: "Thirsting for revenge, the dragon charged at Beowulf and sank his poisonous fangs into his neck." I kept the same meaning but used synonyms to express the idea more vividly.

7) The seventh sentence can be rephrased as: "Beowulf is portrayed as a resolute leader, respected among his friends, a great chieftain, and a courageous warrior-king." I used synonyms to convey the same description of Beowulf.

8) To rephrase the eighth sentence: "In fact, Beowulf is capable of spending days underwater and defeating winged, fire-breathing monsters that dwell both underwater and underground." I used synonyms to rephrase the sentence while maintaining the original meaning.

9) The final sentence can be rephrased as: "The epic poem recounts both historical events and tales of the supernatural and mythology." I rephrased the sentence using synonyms to convey the same information.