This is actually much easier with engine, since the "register_touch" function allows you to input the two classnames.
So we don't have to examine the two touchers - we know they are a player and a nade.
PHP Code:
#include <amxmodx>
#include <engine>
public plugin_init()
{
register_touch("player","grenade","on_touch") // call the function "on_touch" when a grenade and player collide
}
public on_touch(id,nade)
{
entity_set_float(nade,EV_FL_nextthink,halflife_time()+0.01)
}