It's pretty straightforward with factory striders: you throw it at the strider just after your eagle airstrike, and around the same time as all the other stratagems your teammates are throwing at it

More seriously, throw it between its legs (safe bet, will be hit by the explosion) or on the opposite side to your destroyer, so the shell hits its body (might miss, and who has time to look up at the destroyers when getting shot at by a strider).

On a side note, you can disarm the strider (lowering the pressure on the team) with just support weapons. EAT/Quasar/RR can take out its back turret (in two shots I believe), and AC/AR? can take out its chin turrets (takes several shots, and the hitbox is naturally smaller than the back turret)