You can link attacks among the three types of units as long as they are the same color: Core, Elite, and Champion. I will use the Elven units for my examples. Let's say you charge a Treant champion unit. It will take five turns to attack. To link Treant's attack, you must charge an Elite unit or a Core unit when the charge counter on the Treant unit matches the time for the unit you wish to link. So, if you want to link a Deer elite unit, charge the Deer unit when Treant's counter reaches three since the Deer takes three turns to attack. To link a Bear core unit, charge the bear unit when Treant's counter reaches two since Bear units take two turns to attack. The Bear unit in turn will also be linked to the Deer unit. For another example: Let's say you charge a Druid unit. It takes two turns to attack. To link the Druid's attack, charge an Archer core unit when the Druid's counter reaches one. I hope this makes sense. Remember that linking any unit requires that they be the same color.