I have been focusing on magic but I come across enemy archers that wipe me out. The enemy armies seem to have way more armour, so should I perhaps forget about magic and concentrate on researching warfare stuff?
Go for the other two tech trees. In general, play a faction that suits your own playing style--so if you want to delve into magic a lot, Magnar's probably not the best choice.
If I take on a champion that has been killed a few times and they get negative traits can I get them removed?
There is one weak quest in which you face a butcherman, and acquire a single potion that will cure negative traits. That's it, in the game. My advice is to save before battle.
I read on the forum that the more champions you have reduces the viability of each of the individual champions so is it better to have just the one + your starting hero (which is what I have been doing) or maybe only your starting hero?
No: the more champions you have in a single stack, the slower they'll gain battle experience. That said, pairing them off and adding summons or far worse, city troops just isn't always a viable option, not when you're faced with a lot of medium, strong, and deadly opponents. Keep them together, at least for a while, and get someone with Air 2 to cast Tutelage on your champions and sovereign. (Keeping them together will help you develop a sense of how their abilities feed into one another, as well.) Also, choose a few experience-enhancing traits.
I don't touch my taxes and leave them on low, I never seem to run out of money so is this a good strategy?
Depends on how you play. Some people get lots of unrest reduction buildings, and then ramp up taxes to medium. In general, more of all resources helps. You can buy mounts (once you've researched them, and built on their sites), and even buy things if you're in a non-aggression pact or alliance with an AI opponent. If that opponent is Pariden or Pariden-customized, and they're well into the midgame, they may even have spellbooks on sale.
Is Magnar a good champion for a beginner?
For what little my opinion's worth, I think the slavery mechanic drives a need for war, and there's much more to learn about the game elsewhere. I'd suggest Gilden or Altar. Play on the easiest settings, both for the game's overall AI, and the opponents AI. Choose just two or three opponents, and go with a medium or large map. Get a feel for how cities build, spells work, resources are gathered, and the monsters you'll face. Be sure to read the lore in the Hiergamenon--that's the in-game help system. And you can double-click on any monster stack in view in order to get a screen discussing the creature, offering all its stats, and traits.