There are 2 great ways you can use Baron Geddon.
OPTION 1 (available to any golden legendary)
Since you got a Golden B.G. you could disenchant him for 1,600 dust which would allow you to craft any other non-golden Legendary of your choice (if you are not a huge fan of him).
I believe most people consider him to be one of the better legendaries, but above him yet is still (definately) Ysera and Ragnaros the Firelord. Leeroy Jenkins can be a great surprise game-closer as well.
OPTION 2
Second, you build him into your decks (some of them).
I do not have one yet, but I have had him work well in 2 arena decks where I had the good fortune to draft him. He is best used in a late-game style control deck.
I believe the best way to use him is in a Mage deck with +Spellpower (2 kobold geomancers, 2 ogre mages, & 2 azure drakes, +Malygos?) and a lot of spells. Arcane Blast, Blizzard, Flamestrike serve to wipe the board, especially with the benefit of spellpower.
In that kind of deck, Baron Geddon functions as an additional board wipe that also puts out a ton of pressure on your opponent's health.
He could be good in a druid deck because you could get him out earlier, also druids can be built with a lot of spells, similar to the mage and he could add additional control. He probably is a little worse in a Warlock deck than in most others because he competes with your ability for your limitted resource of health. He is probably not great for a shaman or paladin deck because he limits the functionality of your ability.
He will not work as well in an aggressive deck that uses a lot of small 2 & 3 cost minions. Try him out in a few different situations and do some unrated games in "Play" mode and report back here and let everyone know what you found out.
TL;DR: My opinion
Mage: +++
Druid: +++
Priest: ++
Warrior: ++
Rogue: ++
Paladin: -
Hunter: -
Shaman: --
Warlock: ---
The key concept at work here is that of Card Advantage. The basic premise is that cards are extremely valuable, and having access to more of them than your opponent is a strong advantage.
Perhaps counterintuitively, card advantage is more important than mana advantage (especially in Hearthstone where mana is pretty normalized), and card advantage is more important than life advantage (especially in the early game, when a few life points doesn't affect anything). If you think about it, one minion card can be worth 10 life points if it gets some good attacks in. A spell card can also be worth a lot of life, if it kills another creature which can potentially wreck you.
Framing your examples using card advantage:
Elven Archer
- Option 1: Play on first turn, then get steamrolled by a 2/2 or killed by damage. Net effect: Gain 1-2 life advantage, but lose 1 card advantage.
- Option 2: Play when it can kill an X/1, then maybe even attack or block before dying. Net effect: Break even on cards (though note you may have sniped a far better card than your 1-drop), as well as maybe gain some life advantage.
Loot Hoarder
- Option 1: Burn the coin to play turn 1, then die to a Rogue. Net effect: Break even on cards (due to the deathrattle draw), 2 damage to the Rogue.
- Option 2: Save for later, try to block an X/2 or sneak in an attack. Net effect: potentially up one card (you lose the hoarder, draw one, but you killed their minion and/or made them burn a card) and some life. Worst case is the same as option 1. And you keep the coin!
Now of course there are exceptions, and there are aggro decks that eschew long-term card advantage in favor of agression. But the theory there is they are setting up a "ticking clock" that the opponent is forced to deal with, probably by making non-ideal decisions as far as his/her card advantage.
In general, the main idea is: Cards are powerful resources, far more than a few points of mana or damage. Treat them as a scarce currency, and spend them wisely to get as much bang for your buck as possible.
Best Answer
Generally speaking, Hearthstone cards resolve in order of play. As passive cards are cast at the beginning of the game, a First Aid Kit will always be played before a Baron Geddon, so the heal effect will happen first.
This means that one- and two-health minions will die to a Baron Geddon played while a First Aid Kit is active, as opposed to the opposite case, where they would survive.
For more details, you can read the advanced rulebook.