Firo's suggestion is best I think, but alternatively:-

1) Swap attackers or other weak tacklers for yr defenders. OR

2) Before the game starts, give each of your players someone to mark but don't mark any of the opposition attackers. Then when you want to let the opposition shoot, switch formation to mark mode, and hey-presto their attackers will be unmarked!

(Ps You can use Method 2 on any opposition player you want to leave alone to use a tech so you can copy it; ie mark everyone except that player.)