Ok here are tips that have served me well:
1. Explain everything. If you make a move tell them why. "Ok I'm bringing my falcon up so it can attack next turn because it's Fast see?" That gets them to ask "What can I do to prevent that?" and lets you go into how to counter techniques, and when those counters work they also make your defeat more assured.
2. Build your mana needlessly. Cast Mana Crystals/Flowers for no reason. It's a good tactic to a point normally but there comes a time when you should be attacking and not building. Just make sure they start with attacking while you're still putting up infrastructure.
3. Go through their spell book with them before hand. "Oh Thorg here? Yeah he's awesome you wanna get him out quick." Tell them basic good advice before you even get started. Warlocks are awesome for this as "Go in there and set everything on fire" is a simple bouncing ball to follow. Likewise with a Beast Master "Have the Grizzly go murder peoples faces" is also pretty simple.
4. Attack their creatures, not their mage. Also attack their scrub creatures as opposed to their big threats. Kill Asyran Monks instead of Brogan. This lets you do them damage but keep their big guns in the fight.
5. Check to see if they have any questions every so often. A lot of times new players won't cast something awesome cause they don't know what the traits mean. Elusive? What's that? You mean I could have ignored your guards the whole time?
6. Don't cast the obvious counters to their tricks until later in the game when they have a solid lead. As above for example if they got guarding down pat then keep the Mongoose Agility in the book for a while. They're swarming? Don't put on a damage barrier just yet.
7. Teach them the joy of Wands and don't Dissolve them. Let them get the most out of their attack spells by spamming them on an Elemental Wand. Let em keep Teleporting you around with Mage Wands.
8. Cast weaker creatures than them and send yours in to die. Beast Master is good for this, especially against a Priestess using Knights or a Wizard with a Hydra.
9. Bring your mage into the fray early but don't attack much. Bring your mage forward and just cast equipment on them. If you do attack then go for their Mana Crystals and other secondary stuff. Attack a Mana Flower so you can explain Regenerate.
10. Don't even think in terms of strategy for yourself. Instead show off what makes the mage you're playing cool and let it go at that. Slap a curse or two on em if you're a Dark Mage, then explain why Dispel is your friend for example. The rest of the time just keep repeating that lesson after they've got down how to deal with it.
Your mileage may vary but these techniques have worked for me pretty well in the past.
