I am assuming 5 marines are in same unit. Each single marine's shooting is sorted out independently, but you have to declare the targets and the split fire for the whole unit before any attacks are actually made.
Following this, in the picture presented by you none of the marines will be able to shoot at Warlord while G1 remains alive at start of shooting phase - All the shots need to have declared (legal) target already before G1 can die, and thus no one of the marines will be able to target Warboss legally.
For you to be able to do this thing you are asking, to shoot at warboss, you will need to have some marines in different squads, so they are activated after you have shot Grot 1 with one of the squads.
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