Due to [BT9-103 Kongou]'s effect, cards can't be placed in security stacks, therefore this card's "place 1 of your opponent's Digimon face down at the top or bottom of your opponent's security stack" can't be activated, and the target Digimon will be left as-is
Really, the ONLY way to kill it is to fling a monster with 2800+ ATK onto the field quickly and run it over, which can be difficult to do, and even more so when the Alien player can keep recycling Fiendish Chain to block attacks
Cause thats just what mega Pokemon do on evenings off of training.
The other option is to make another monster Level 4 to match Pegasus, which allows you to go into either Loading
4, 2025 What does "none of your [...] Digimon are affected by your opponent's Digimon's effects" mean, exactly