If this card attacks a Defense Position monster with DEF 2000 or more, destroy the monster with this card's effect without applying damage calculation.
Rulings
Individual Card FAQs
You destroy a monster with "Disc Fighter"'s effect during the "calculate damage" portion but before the "apply damage" portion of damage calculation; which effectively means it's right before the actual "damage calculation".
When "Disc Fighter" attacks a face-down monster, after it is flipped face-up you can use "Micro Ray" to cause the DEF to go below 2000, and the monster will not be destroyed by "Disc Fighter"'s effect. Or, you can use "The Reliable Guardian" to increase the DEF of a monster above 2000 so that it will be destroyed with "Disc Fighter"'s effect.
The Defense Position monster is destroyed by "Disc Fighter"'s effect, not by damage calculation or "as a result of battle".
If "Final Attack Orders" is active, a monster attacked by "Disc Fighter" and flipped face-up will shift to Attack Position, so it will not be destroyed by "Disc Fighter"'s effect.
EARTH|
Level 4|[ Warrior / Effect ]
ATK 1000|
DEF 1000