Core: Dragon Knight is one of the simplest and most effective heroes in the game. He's pretty tanky and has high armor and regeneration. All of his spells are simple and to the point and there are no complicated spell combos to memorize. A key item for him is the Soul Ring, which solves his mana problems (it is an item that sacrifices HP to gain mana). Because DK has such high HP and regeneration, you can pretty much use Soul Ring without worrying about getting yourself killed.
Support: Lich is one of the easiest support heroes to play. Good nuke, decent move speed, and has no mana problems thanks to his signature ability, Sacrifice. Sacrifice instantly kills an allied creep and gives you mana based on how much its current HP was when sacrificed. Also, anyone within XP range gets the creep's XP. If you sacrifice creeps away from the enemy, you not only gain extra XP, but you also effectively steal XP from the enemy. Sacrifice also pulls the creep wave towards your towers, forcing the enemy to move further from their tower in order to gain lane XP.