For any player wondering if it is possible to clear the colosseum within the 48 hour time limit the answers is yes.

However the requirements are awful:

• Have access to tower rank 3 or better due to the points required - about 98,000 all in.
• Your own health pool is 80,000+ to survive a lucky first strike by the defender. I'm level 47 with 83.6k health.  There were a number of attacks that dealt 50-75 thousand damage in one turn when facing rank 3 opponents;  this was without them unleashing their active abilities.
• Have a balanced spread of potion bonuses to maximise your damage output. I had 5x +65%, 1x +85% for damage and +95% to health.
• Possess an extremely large card collection outwith the common cards (double drop rate and +1 gem level commons will be wanted, the others I generally ignored). You're going to be playing 50 or so hands in one session before you run out of hands that can win quickly and earn a decent amount of points - i.e. ignore the rank-4 fights. My unused cards were generally common cards or had less than 100 damage on their profile. I currently have 389/430 cards aka 90% of the deck with most of my cards being level 5-10. Only my world boss fighting deck has been heavily invested into.
• Be willing to switch to medium difficulty opponents when necessary in order to protect your 3x multiplier.  I lost about 6 times through a mixture of being cocky/over confident and unfavorable drop rates. The latter I couldn't avoid.
• Have 12-14 hours spare to attempt the challenge. I was on the 7th cycle of my deck when I cleared everything, so about 330 -340 matches were fought.

You won't get a steam achievement and the on screen confirmation of your accomplishment is shown below. You CAN still fight arena fights afterwards to allow you to farm for event silver and card top ups.