i guess as machaka you have no chance .
but first some further questions :
-at which turn are you ? 40 or later ?
-how many provinces do you have / your friend ?
-does he have all mages in one province ?
perhaps in this special case you could have some success with several seeking arrows + flames from the sky or if he has a high upkeep
perhaps you are lucky with bringing utterdark up and keeping it some turns . but depends how evil he clamhorded .
cause if he clamhorded enough + has the alchemist stone which i suppose he will be able to pay his upkeep by alchemizing fire gems .
both are not very good suggestions
now the ihmo most important question :
his mystics can cast accahsic which he most probably did and so he surely has found every magic site + clamhording .
now did you yourself clamhord ?
how is your gem income compared to him ?
if you suppose you have only half or even less of his gemincome then most likely you have almost no chance .
i guess this is the case
so finally ihmo the only chance you have :
hope he has most of his mages + his pretender in his capitol . try to find this out .
if it is the case : astral travel / teleport as quick as possible to that province and hope to kill his pretender .
if your friend keeps his items and really dies they are all lost + 1 magic level in every path.
furthermore hopefully he has no second empowered enough mage and when you then can keep the capitol even if he recalls his pretender he is summoned in his capitol and thus dies again .
but in your next game :
either take a smaller map and play it 1on1 with him without computer opponents.
or if he again takes arco which is ihmo shortly before ryleh / pythium the best nation for the tactic your friend used then take ryleh or pythium and try to be as good with them as he .
or try to rush him early on but on a large map this is hard .
if you play ryhleh in your next game you have the advantage that you are for the first time quite save in the water + hopefully there are many water sites so you have a clamhording advantage .
then in midgame try to find his researcher/clam province ( i guess it will most likely be a province with sages ) and try to kill them all in a suicide attack .
if your friend is clever enough and not to lazy to spread them then this is not good
and after the first time you used this strat successfully on him he will not make the same fault again anyway .
Last small idea :
get a nation with spies and try to instill uprisings in his best provinces .
if he doesn't search/find your spies via pd but via patrol at least he kills some of his ppl too.
if made early enough it can perhaps do some damage . + try to kill his fragile mages with seeking arrows when you have some air income + some indy mages who can do this .