.hack//Mutation - Virus Cores D, E and F

Firstly, you need to know a little something about data drain. Monsters ususally have three things they can be data-drained for - a virus core, a normal item and a rarer item. You have a higher chance of getting a rarer item when your data drain is closer to 100%, and a better chance of getting the virus core when your data drain is closer to 0%. So, for instance, if you DD a Wander Demon, you're more likely to get the Rod of Pattern when your DD is red, and more likely to get virus core F when it's blue.

The other thing to know is what Helba told you; that virus core D comes from small monsters, E from mid-sized monsters and F from large monsters.

Lambda Shapeless His Sandtrap is a good place, I found, for virus core F. Wander Demons can be found on the field, and assuming you're a high enough level to do this easily, your DD should stay close to blue. Mad Witch's will give you virus core D and SWORDs.

Lamba Bitter Astigmatic Treasure Gem I found was a good place for virus cores E. You'll get virus core E from most of the monsters, but a few Ds from the little mages. Hit them with a basic spell twice, and they should get protect break status (use the party menu to tell your other characters to attack something else while doing this).

Before attempting either of those, I suggest running through another level 50 dungeon (I liked Lambda Discovered Hot-Blooded Corridor) on a weapon and level hunt. Other FAQs detail which monsters carry the best rare weapons for your characters. Here's another hint - get a Rod of Pattern from a Wander Demon asap (you'll have a mage with you, of course - they make levelling so much easier).