I ragequit on my mage run when my geo / pyro + summoner / hydro ran into fireslugs. Everything I do explodes and heals them... ._. And the long cooldowns / low damage on water skills make fireslugs' weakness to frost negligible, as I can't kill them before they heal to full from fires all around them.
I almost misinterpreted that to think that you had one character running geo, pyro AND summoner, but it was just geo and pyro, with the other one being summoner and hydro. I guess you forgot about the fire slugs?
Okay, well obviously you'll want to give your pyro a third attack type, because as you realized, the two you have aren't going to work well together there. This was the case in D:OS 1 as well. You need a third element as backup.
I'd make the pyro a summoner as well, because if nothing else they can cast Dimensional Bolt and/or Create Totem very turn. Take Dimensional Bolt and use it in areas not likely to be set on fire, and make air/water totems and incarnates, or just plain wooden ones for physical damage.
As an aside, the fire slugs are totally optional. You don't need pet pal, you don't even need to fight unless you get near the queen. The only one who you can't sneak past is in the back and if you're polite, they just let you go through. I almost always leave the fire slugs alive because they're nicer than the magisters.