Well control decks also use combos to kill other control decks, nothing really out of the ordinary, but you could argue combos are too strong compared to life total I guess.
Could nerf Leeroy, you're still going to run into combos that do 15 to 20 dmg though even with a leeroy nerf, double savage roar force of nature with innervate is 22 assuming no other board, shaman alakir+rockbiters or leeroy(even nerfed)+windfury+rockbiter is like 20+ easily and any board with 4cards and bloodlust+windfury is 15 even if the mobs have 0dmg, Velen+Mind Blast is only 10 but rough if you don't instantly remove it, Warrior can still do weird combos like Warsong(or Charge)+Worgen+Cruel Taskmaster+Rampage for 18, Mage can Fireball+frostbolt+icelance x2 for 17, Hunter combo is reliant on how many mobs you have but buffing wolf+unleash the hounds with 2tokens+2x kill command is 14, Warlock is potentially stupid damage assuming you have one charger and 2 overwhelming + 1 soulfire regardless of the charger(it's 14dmg with the 3 spells to begin with so charger can be a 3/1 orc and that's still gonna do 16), Rogue I'm too lazy to do the math but you know how it works.
Obviously Leeroy doesn't help in those combos balance since it's generally the most efficient card you can play especially if you have a 1mana buff and a faceless or a way to attack multiple times with leeroy(windfury for 2, shadowstep for 2). Nerfing Leeroy to 5 might help a bit with some cases, you're still gonna get comboed though.