How to take the enemy capital in MP

[Saxon]

So, your war goes well, you have rolled up your enemy and only the capital is left. It is a MP game, so you do not have time to dominion kill the player, you need to fight. You also really want those gems sites and income. The problem is, it is turn 40-60 and they have 30 or 40 mages sitting there. It was a big part of their research effort and some of the mages are capital only and pretty good. Any attack is going to face a storm of high level spells.

What do you do? This must be a fairly common situation, so how do the experienced MP folks handle it?