Trample over planeswalkers means that when Thrasta, Tempest's Roar deals combat damage to a planeswalker it's attacking, it can assign any amount of excess damage to that planeswalker's controller. Excess damage to a planeswalker is damage greater than the number of loyalty counters that planeswalker has. Deathtouch does not cause damage dealt to a planeswalker to be lethal.