The motive of that was to squeeze out more energy to deal with those High Elves while hoping that the fake Highlord could use up more energy of Lin Li's artifact. It would be even better if both of them suffered serious injuries.
Although Donald had never seen a real artifact before, he knew that its user would suffer huge consequences if he didn't have the level of abilities that matched that of the artifact. For example, while the Magical Crystal Cannon was very powerful, its consumption of magical crystals was also immense.
