Subtleties of global spells

[En Forcer]

I'm in a SP game playing MAN and here around turn 190 it's just me and Pangea (with the water folks floundering around uselessly).

We've gotten into a war of casting global spells. I dispell his as fast as I can but then I get one up and it gets replaced or knocked off the list.

I understand there is only room for 5 at one time (which is way too little in my opinion) but my question is that sometimes I see the same spell for both of us up at the same time, and other times, he just replaces my spell.

Do I need to have more gems invested in the spell to make it harder to replace or keep it from being knocked down (once over 5) or what?

I would appreciate any insights you guys have into global spells. This is getting really frustrating!