No matter how well the AI is optimized, it won't be able to play every single deck as effectively as it can. When it is programmed to wait for combos, it waits for combos that never occur, and vice versa.
My idea here is to make it so that you can adjust some constants of the AI to make it play cards as quickly as it can, for example. When you build you arena deck, you could open a window where you could choose constants, and change them.
Examples of constants:
Aggressiveness (The AI puts more effort into defeating the opponent rather than preventing its own defeat the higher this is)
Randomness (The AI acts more random the higher this is)
Patience (The AI waits for combos more the higher this is)
Riskiness (The AI makes more and bigger risks the higher this is)
Reaction (The AI does effects that counters what has been already done more the higher this is)
Also, this could give the main screen AI opponents some more "personality" such as Hecate being more aggresive than Jezebel, for example.
Possible problems with this idea:
What exactly will it be waiting for when waiting for a combo? (Example: Playing deja vu, does it wait for a Blessing/Momentum to show up before it splits the deja vu or not? What about Chaos Power? Or Adrenaline?)
Some of these variables could be split into multiple variables (such as randomness become random creature targeting, random playing, etc.)
Some of these variables don't exist, make no difference, or always are better one way than the other.
