The max coupons you can get from the snake is 40 per pop (80 total). If you are soloing and not getting 40 coupons per, try partnering with a groupmate as this should give better rewards (i.e you all only have to run 1 barrier ninja, and global buff ninjas will help everyone - eg: mifune for LMC). Also doesnt hurt to a strong groupmate that can easily carry both of you to the 40 coupon damage threshold, the weakest ninja in the group should run the barrier ninja, rest maximize damage.

Only downside is the increased fight time of 10 rounds of fighting for 3 players, but thats the price to pay if you want coupons and cant get it alone.