Team up from the beginning(before you join the mission!). So you can share the buffs and coupons, coins from the first wave(in a 3-man-team all 3 guys get a buff). Stay in team and attack the serpent. After 50%hp of summon if anyone retreat or joins the summon instance, the Ambush will be activate. Killing them gives 5 coupons and some coins to each person in a team.

For anyone who wants the top 3 spot in summon fight, do it solo and wait for a team to make the path save. The others c*ways farm the Ambush as a team.

For a small group which struggle with the encounter, it would be wise to do this in a 3-team always. It would be even better if guys buff each other(fitting barrier, etc.).