So, it has occurred to me that apprentice mode is very different from arena. It's almost as different from arena as academy is. But Arena is a complicated game. How do you teach it to people without overwhelming them with analysis paralysis, and ALSO without drastically altering the rules and gameplay like apprentice mode does?
1. Use full sized 3x4 zone arena, 120 pt preconstructed spellboooks, and use the ability cards. Have damage start at 0.
2. Have a five round limit. At the end of round 5 the demo ends. If the players want to continue after that and there aren't any other potential new players waiting, they can continue. Otherwise they're done.
3. First two rounds you choose their planned spells for them and walk them through the basic rules and the stages, phases and steps. If it helps make things less confusing, have mage action markers and quickcast action markers start in the used position. That way you don't need to skip the reset phase round 1, so it's more consistent and less confusing over all. In particular, don't forget to explain:
-the mage stats and abilities
-the components of a card, that all spells have a mana cost, an action cost, a range, a target line, and a level.
-the difference between quickcast actions, quick-action spells, and quickcast action phases.
-the difference between actions, action phases, action costs, and activating a creature.
-the different types of spells.
-the difference between spells and objects.
4. The remaining three rounds after that is free-play. During the planning phases of the free-play rounds, tell the players that they should look at the cards on the board and in their spellbooks while asking themselves the following six big questions.
SIX BIG QUESTIONS OF THE PLANNING PHASE
-How do I plan to kill the enemy mage?
-What obstacles are there to killing the enemy mage that way? What can I do to deal with those obstacles?
-What might my opponent do to try to stop me, and what can I do to prevent or counter that?
-How might my opponent plan to kill my mage?
-What can I do to stop them or make it harder for them to kill my mage?
-What will my opponent do to prevent or counter that?
Make sure to give them examples of what they could do to help get them thinking.
Try to answer any questions they have as they come up. Do NOT try to explain any key words that do not come up in the demo or that the players do not ask you about. There will always be more and more key words in the codex over time, but the fundamental rules and main gameplay mechanics will always be the same. You'll still need to explain key words, but only the ones that actually become relevant during the demo.