I promise- you don't want to join a clan that filters players based on level, thats very short sited. Level has absolutely no relation to troop levels.
But yes, donating and upgrades are by far the fastest way to earn XP.
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Drop trophies below crystal if you're up there or higher, and snipe th like mad. 10 xp per snipe I believe, and costs very little as far as troops go.
What clan?