The AncientGreymon is showing its true value on tournament when the promo card [P-029] Agunimon, a hybrid type digimon, is introduced on Mar 2021 for First Year Anniversary. We can evolve AncientGreymon into this digimon with only 2 cost when this digimon attacking. To serve the purpose, when you evolve this digimon into AncientGreymon, it should be still in your turn to do SecurityAttack+x to your opponent's security, because we must delete this digimon at the end of the turn. The deck for AncientGreymon and [P-029] Agunimon must include the [BT4-009] Flamemon to search for red tamer and [Hybrid] digimon mon in top of the deck to put to hand, the [BT4-011] Agunimon to add more red LV4 Hybrid digimon cards to deck, so that when the AncientGreymon got destroyed we can play a LV4 red hybrid from hand. Besides, the BT4 Agunimon can evolve from a red tamer, so it helps to boost the robust for the deck.
To get more detail of this combo, you can refer to this post "New Promo Agunimon Hyperfast!!" for a deck strategy.