Even the level 3 Dao Heart that he was pretending to have was enough to block the attack of 10 units. But if he truly had this level of a Dao Heart, his mind would be preoccupied with defending against the mental suggestion and wouldn't be able to defend against Slav's subsequent attacks.
So it was reasonable that when Slav tossed out the grenade crafted with the Throwing Money Metac, he didn't have enough mental power to jump away and dodge the grenade. All he could manage to do was to roar in defiance and rush headfirst at Slav.
The path he was taking to Slav meant that he would crash head-on into the grenade. But he didn't stop because he was sure that the grenade wouldn't be able to breach his defense.
