It seems to me that Easy Mode is the best way to get XP in Kaos Mode. I'm not 100% sure the it gives more than other modes but even if it doesn't it is still the best way.
It is the best way because enemies so very low damage to you which means you can easily defeat all the enemies which means quick XP.
Only the last three of Kaos' Doom Challenges will let you level up faster. Suggestion: If you have already captured Kaos, evolved him and have him inside the Kaos Trap (regular or Ultimate), use him and perform the Undead Doom Spell attack to kill many enemies, including large ones. But make sure you don't kill them all at once. Just defeat at least two medium enemies or one large enemy at a time, and then absorb the XP.