This is the beauty of Ulduar -- if you want prestige (and the best gear), you have to earn it by flexing your raiding muscle and beating encounters on their respective hard modes. If you don't have the chops or the time for the highest difficulty content, then you can still beat the instance and see every encounter.
But, of course, you want those 239-level items.
So you should probably read after the jump and find out how to activate each boss' hard mode, yeah?
• Flame Leviathan: Speak to the Lore Keeper of Norgannon first and select his gossip option, then speak to Brann. Leave one to four large colored towers up to have Flame Leviathan's HP, damage, and abilities scale. Four towers up is the hardest mode.
• Ignis the Furnace Master: Ignis has no hard mode.
• Razorscale: Razorscale has no hard mode.
• XT-002 Deconstructor: When his heart becomes exposed, destroy it. This is a DPS race. When the heart is destroyed, XT-002 gains 50% more HP and does 30% more damage. Destroying the heart also heals him to 100%.
• Iron Council: This encounter scales depending on what order you kill the NPCs. The NPCs will gain new abilities and do more damage depending on who's left up with them. Leaving the dwarf up last is considered the easiest, followed by the vrykul for medium mode, and the giant being the last add is considered the hardest.
• Kologarn: Kologarn has no hard mode, though it's significantly harder anyway if you kill both of his arms.
• Auriaya: Auriaya has no hard mode, so far as I'm aware.
• Hodir: Hodir's hard mode is simply a DPS race. Kill him before three minutes are up to get both Rare Caches of Hodir; kill him before six minutes are up to get just one. He enrages at nine minutes.
• Thorim: Battling through the Arena and engaging Thorim within 3 minutes causes Sif not to despawn and assist Thorim in the fight. Sif cannot be targeted and will nuke the raid.
• Freya: Just like Sartharion -- leave her Keepers (up to three) up during the fight. She gains new abilities for each Keeper -- just like Sartharion.
• Mimiron: Have someone sneak behind him and hit the Big Red Button. Pressing it activates an 8-minute enrage timer and increases the health and damage done by the vehicles in the encounter by 25%.
• General Vezax: Don't use the Saronite Vapors to regain mana. If no member of your raid makes use of the Saronite Vapors, then after 8 Saronite Shards are dropped, the Saronite Animus will spawn. It's got 8 million health on heroic and applies a debuff to the raid that increases shadow damage taken by 10% every 2 seconds. Kill it and kill General Vezax for your hard mode reward.
• Yogg-Saron: During the fight, if you've defeated any of the Watchers (Hodir, Thorim, Freya, Mimiron) before engaging Yogg-Saron, then they'll assist you in the fight, but only if you've talked to them outside their hall before you engage Yoggy. Don't activate one to four Watchers for scaling loot. The meta achievement requires you leave one or fewer Watchers up.
• Algalon: Algalon has no hard mode -- he IS hard mode.