  This paper describes the heuristics used by the Shadoks team for the CG:SHOP
2021 challenge. This year's problem is to coordinate the motion of multiple
robots in order to reach their targets without collisions and minimizing the
makespan. It is a classical multi agent path finding problem with the
specificity that the instances are highly dense in an unbounded grid. Using the
heuristics outlined in this paper, our team won first place with the best
solution to 202 out of 203 instances and optimal solutions to at least 105 of
them. The main ingredients include several different strategies to compute
initial solutions coupled with a heuristic called Conflict Optimizer to reduce
the makespan of existing solutions.
