A is the last challenger in the first half of the game. By this time, Infinity Tower will be discolored, and there's no way out. All you can do is to talk to Rosemon to open the battle arena and also updating your Deck.

When updating your Deck, you must have these: circle counter-attacking Digimons, HP-adding support cards, counter-attack support cards and boost-up AP support cards. Why these? I will explain it to you.

In your battle, A will send your Partner cards to the bottom of the Deck. So you might have no chance to use them and Armor Digivolve. If you have Partner Finder in your Deck, you can use it but sometimes to no avail.

First he picks DemiDevimon and then Download Digivolves to Ultimate Apokarimon. Apokarimon has high HP and also high AP. Then you have to pick any Digimon that: either reduces circle AP to zero or counter-attacks circle. You also have to increase your Digimon's HP or your Digimon will go to waste if Apokarimon attacks first without a circle.

However, A usually uses circle attacks for Apokarimon. So, using a circle counter-attacking Digimon will cause the attack to hit him back. You can also use Hacking cards to exchange HP with him (if you're using Rookie or Champion Digimons). However, do not use Hacking when his HP is lower than yours.

Oh yes, when updating your deck or creating one, make sure that your deck is NOT A NATURE-BASED DECK because DARKNESS-BASED DECK DESTROYS NATURE-BASED DECK. A Fire-based Deck or Ice-based Deck is recommended.

Use this strategy to defeat Apokarimon, and finish the other two Digimons of A by boosting your AP. Try it, it works!