The spell you are thinking of is earthquake, a 6 pip myth spell. You can purchase a Crown robe in Mooshu that has an earthquake card. Cheapest is brown/brown for boys and white/gold for girls.
The trick to killing Mavra is to kill the pox first. The pox will just keep shielding her and themselves, so if you kill them, she is a lot easier. If you are Balance, load up on Hydra cards which will kill 3 shields at once. Also make sure you use plenty of blades and traps. The pox will not attack, but they are a real pain nonetheless. Try seeing if a friend can help you.
I hate to tell you, but there is another that is nearly as hard shortly after Mavra ~ Sandor Spearcaller has 2 protection pox that will continuously blade him and use the 50% ice shield that shields against all spells.
If you are going in solo, I'd suggest ignoring the pox and focus everything on taking out Mavra as quickly as possible. Balance wizards can ignore her shields. Death life drainers can ignore any absorbs. Fire can DOT her over and over, sneaking in to strike when her shields are down. Myth can try Earthquake followed by a big damage spell. And Storm can just hit her with maxed out spells, blasting thru her shields/absorbs. I highly suggest casting a minion to add dps. Once Mavra is down, just take out the pox without worrying about healing.
Ice and Life might have to bring a friend if the duel is taking too long.
I agree with you this is really hard, but it is possible. I am a life wizard so I think it was a little easier for me since I can heal myself but, nonetheless, if you concentrate on one school of attack and take out the pox first, you should be able to do it. Good luck and keep us updated on when you make it!
Thanks for all the tips. I rearranged my spell deck using more smaller spells rather than the big ones, didn't worry too much about shielding, used by guardian sprite to keep me alive and brought my daughter's level 17 character along for help. It is easier if you just focus on using one school of spells.
Just beat Marva, took me almost an hour, all by my one. Then took out Sandor Spearcaller in ten minutes, very simple. Also defeted the Kraken and the Mossback to finish Plaza of Conquests. I have done all side quests as well so now i am going for the Grand Chasm.
I focus solo are marva, same with the other guy you can take out the pox rather easily after the boss is dead. Never had any problem with either of the on my life mage. Member to wear the sword of you primary and bring your rank one spells too, doesn't hurt to pull your minion out either.