What I was saying with the /assist thing is that for the period of your ping, you can select a target and if you hit /assist, your toons will pick up your previous target. That may or may not apply to your situation.

As for /targetlasttarget, there is nothing in ISBoxer by default (from the Quick Setup Wizard or otherwise) that does this, you would either have to have this in a WoW Macro Action (by entering /targetlasttarget, or selecting "... and restore Target afterward" in the "Targeting helper" box... or if it's not either of those, it would have to be something in WoW itself -- either a macro with /targetlasttarget, or a Key Binding in WoW's Key Bindings interface set to target the last target.