The opponents in expo are based on your top 5 hero's
Stage 1 the enemy will have a power level around 85% of your top 5
Stage 2 in the low 90s
Stage 3 around 100 %
Stage 4 105 to 110 %
Stage 5 115 to 125 %
Overall having high power level hero's who don't fight as well as they should make expo harder. Having a max level fully enchanted dark Drac with a power level 15% higher then your next best makes your expo opponents that much harder.
If you have that situation then you will just have to wait and build up other hero's until you get legendary Medusa.
I had your problem on server 8 back in November. I could finish expo on that account about 70% of the time on insane.