If there are Spell or Trap Cards, or face-down monsters, on your opponent’s side of the field, “Drillago” cannot attack directly.
If “Drillago” attacks directly, and your opponent activates an effect to reduce one of his face-up monsters to less than 1600 ATK, there is no longer a condition allowing “Drillago” to attack directly, and a replay occurs.
If “Drillago” attacks directly, and your opponent activates “Ready for Intercepting” from his hand, because his/her “Makyura the Destructor” was sent to the Graveyard earlier this turn, to flip one of his face-up monsters face-down, there is no longer a condition allowing “Drillago” to attack directly, and a replay occurs.
Netrep Q&As:
Q: When I attack with “Drillago” and all my opponent has on the field is “Injection Fairy Lily,” if he raises the ATK to 3400, can I attack him directly?
A: You declare your attack during the Battle Step. “Injection Fairy Lily” is a 400 attack monster at the time, so “Drillago” would not be to use its effect and declare a direct attack. “Drillago” decides to attack the “Injection Fairy Lily.” Even if the effect of the “Injection Fairy Lily” is activated in the Damage Step, “Drillago” will not be able to change to a direct attack. “Drillago” will continue battling with the “Injection Fairy Lily” and damage calculation will occur. Curtis Schultz, 03/08/2004