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Reaching the boss is the main goal, since bosses give huge amounts of XP compared to normal enemies, so the player should aim to finish normal waves as quickly as possible and move on to the real goal.
Ideally, the player wants to strike a balance between these two factors, playing on the highest difficulty that they can without dying too early. Higher difficulties award more XP but are obviously more of a challenge.
Endless Mode is one of the best ways to earn XP overall.
