Ok. Hopefully that Vital Throw got Staryu far enough away to prevent the effect of Light Screen on Trapinch, so we're going to use this to fire a Shadow Ball at the exposed insect, courtesy of Iota. Sigma, you concetrate on finishing Staryu. Go at him with another Vital Throw, once again to keep it away from Trapinch.
Then I want Iota to use Confuse Ray on Trapinch, that should mess him up a little. Meanwhile Sigma should Thunderpunch Staryu, or Revenge if he attacks you.
For the final action, another Shadow Ball from Iota to put Trapinch in his place. Sigma, if Staryu is still kicking, put an end to it with Thunderpunch, otherwise use Ice Punch on Trapinch.
Iota: Shadow Ball (Trapinch) ~ Confuse Ray (Trapinch) ~ Shadow Ball (Trapinch)
Sigma: Vital Throw (Staryu) ~ Thunderpunch/Revenge (Staryu) ~ Thunderpunch (Staryu)/Ice Punch (Trapinch)