Keep in mind I am not lvl 50, in fact I am only level 17.
So far it seems that my stratigy is to out live my opponent. I do smaller 'pea shooter' attacks, such as the zero pip wand attacks, damage shields, blades, etc. Then when I know my 'big' attack will be enough to take them out, I go for it.
Another thing is to get some defence up. Something like a 50% shield. I usually don't do it right away, but I let my opponent build up a couple shields, and then lay down a defence, just to throw them off a bit.
I try not to drop below 50% health if I can help it. It takes quite a bit of time to do the big attacks, so if I can keep my health above 1/2 I can usually take a hard hit, and then use the next few rounds getting life back.
If you are on defence, you need to 'play it safe'. Remember that if they can hit you hard enough, they can kill you in the current round, not giving you a chance to heal. Most attacks are easy to calculate. If they have one pip, it will be around 60-150 damage. If they have 2 pips, its around 200ish. etc.
So keep an eye on how many pips they have compared to your health.
I am still new at the dueling. I am only a sargeant, but I have been able to beat a level 28, and almost beat a level 32. He started off telling me he was level 32, and then proceeded to explain that I was crazy, and that he has better spells than I do, etc. Toward the end of the duel he was begging me to stop using 'healing spells' and to just let him win and be friends.
I didn't let him win, he won on his own, but it was VERY close. I don't say this to brag, I am just saying that to duel in this game it is like a chess game. You need to play a few steps ahead, and get into the mind of your opponent. Most people it seems like to come out with their big guns. I find that if I play my cards right, and out live their primary offenses, you gain the upperhand.
One time I was delt a very crappy hand, and got to around 150 health in the first 10 rounds. I then said 'I got a bad hand this time :-(' My opponent just laughed. After that my luck changed, because at the end he was asking me to just let him win so he could go eat dinner. I told him 'maybe in a couple rounds'. Well he got to go eat his dinner in the next round because I won. :-)
One tip I would give, is to not use a wand that is from your same class or your prism class. Such as if you are death, don't use a death wand or a life wand (if you intend on using prisms). The reason is that if you build up your death blades and shields, and then use your wand, now you just wasted them. However if you use a fire wand or ice, etc. It is safe to use without breaking your damage bonuses.
Also, as soon as someone puts up a shield against my damage type, I just cast the next available damage spell. What is the point to try to build up even more, when it will just get reduced to crap? If I have a damage shield and a blade up, and my opponent puts up a 50% shield, I'll use my wand to break that (since it isn't my primary school). If they put up a 75% or 80%, I'll try to use my lowest pip spell to get rid of the shields. If they use a damage absorbtion shield, usually I will take small bites out of it, but leave it up, so they think they are still safe, and then do a massive attack. It only absorbs like 100 out of 700-1000 damage, so it puts them off guard.
Yes, shields are important in arena duels as they can save you from a quick demise; however, since the OP is max level, people his level will have MANY, MANY ways to get around shields. Absorb shields will not help you at this level of play, in fact you can actually lose momentum against a fellow grandmaster playing spirit shield or ice armor when you could have spent those pips saving for a big attack. I'll also tell you right now that death attacks completely ignore absorb type shields and a grandmaster death could just use poison to remove any other shields you put up. Grandmaster ice wizards can also do this using frostbite and fire wizards can use either scald, fire dragon, or fire elf.
If you're going 1 on 1, I suggest you beware of grandmaster balance and myth wizards. Balance attacks are very difficult to defend against since there are no shields that specifically defend against balance, and judgment can hit REALLY hard once they have saved up enough pips. Myth wizards are even worse because as of right now there is NO WAY to defend against their strongest attack, Orthrus. Orthrus not only does 2 attacks, but the first attack ( and lowest hitting) wipes out all shields on the opponent, allowing the second attack to do its normal damage (675 unbuffed I think). For this reason, don't go overboard on shields and blades(earthquake will wipe them out) when you fight myth wizards; heck don't even bother with shields, just attack, attack, attack if you're ever paired against a grnadmaster myth. For every one else, use shields only when you need them, don't go overboard on them or you'll run out, and don't use too many blades at once because if your opponent sees you have a lot of blades and traps out, they'll sense an impending attack and start promptly throwing shields up. lastly, it doesn't hurt to have reshuffle handy in case you deck out, they can be either in your side deck or main deck but they need to be handy; I've lost some battles due to deck out when i forgot to put in reshuffle, so DON'T FORGET!