So what is Pythium's weakness?

[st.patrik]

The question I'd ask (which some people have already said) is not 'what is Pythium's weakness?' so much as 'what is Arco's strength?'. And Arco's strength is incredibly versatile magic - you have strong astral, and strong elemental magic - you can cast over half the spells out there. So you need to know your spells, and use them to your advantage. Pythium won't be able to stand up to the kind of battlefield enchantments that you can ward your troops against (even if it's only partial warding). Of course you do have the Achilles' heel of Mind Duel, which you'll have to watch out for.

Like others have said, watch out for the javelins - but since you don't have missile troops to speak of yourself, get yourself a staff of storms and/or cast mist to decrease their effectiveness

Plus there are lots of fun things you can do with elephants, if you plan right...