I've had so far a very good experience with magic defense with the empire mages, although i was using the standard wizard familiar/lvl4 mage with +5 to dispel. I've had everything from elven mages to necromancers struggle a lot to cast even one spell. Also having more than one mage does not increase your dispell, although if you have it in melee a second one comes in handy obviously.
I was putting the armor of tarnus on the mage since o was giving all three the 5+ save but i've downgraded the captain's to a 6+ so yes i can save 5 pts with the talisman on the general and the cloak on the mage.
Yeah the sword of justice is not ideal, i sas going for the sword of battle or the ogre blade firs but was lacking the pts, i'll try to refine the units a little so that i can give it to the general. The main draw is the multiple wound (2), intended to meet monstrous cavalry like pegasus knights (the big ones are left to the mage obviously). Another option would be the runefang but 100pts is too much for a weapon like that. It should cost 80 pts at most. The dueling blades are nice but i'd rather keep using the shield with the sword of battle than having another attack.
Regarding large infantry blocks, in my experience you don't need to do anything for them. they are weak from the get go, they die pretty quickly to shooting (and the demonology's signature missile is perfect for them), plus demonology has s very good hex for those (-2ld and -2 I, no inspiring presence) and being super slow i have not seen them as a threat so far. The most dangerous are the shooting types, like 40 lothern seaguards with the razor banner, but even then shooting appears to be severely limited in this edition so they have so far only managed to kill a unit of knights, sometimes (in a similar list without the griffon mage and with more units, although i think the HE player uses them not optimally).
The biggest dangers here would be bolt throwers actually, those are deadly but often the pkayers that i have encountered somewhat discard the griffons as lesser threats compared to the big unit of knights or the outriders. I don't know why but i won't correct them for sure!