What class are you playing? It would be easier giving you specific tips if we know what sort of playstyle your character has (is it ranged? Melee? Can you self heal? Etc).
It's been some days since your post so I'm not sure how relevant your questions are anymore but;
1. Kaliga is stronger and has a different attack pattern if you do not kill any other bosses in the instance before entering the final boss room. I don't remember everything he does, but I think he buffs himself with the 2 orbs up by his "altar" and he summons adds at some points of his hp%. I think he also stuns and knocks you down more in this form.
A general tip in pve is to kite out (= run out of attack range of the mob) any casts and aoes, or sometimes even buffs (attack increase, reflect, defensive buffs and shields). Kaliga's stuns and knockdowns are cast skills afaik, so as soon as you see him casting you should run away (or press evade if you're playing a scout class). Even if you get stunned/knocked down you save yourself some time by making the mob have to run to you before it can follow up with attacks.
2. If you want key from Kaliga for weapon boxes, you have to ignore all other bosses in the instance. The key does not drop from weakened Kaliga (which is the version you get if you kill other bosses on the way).
3. Campaign quests are the best way to exp.
4. This might be down to your class, the gear you're wearing, and how you're playing. What are you having trouble with? Are you dying to single mobs? A bit more information would be helpful.
5. There's not really any efficient ways of making kinah as a low lvl player. Starting with DC and upwards, end bosses of instances will drop items (1x for everyone in group) you can sell to any npc for 50k, 100k, 200k (value depends on boss). That's about it for pure kinah sources, you won't be dropping enough trash gear to sell for any significant amount of kinah. Any manastones you come across will be worth saving, though, even the less useful ones like evasion, parry and mp. You can either trade them to the manastone removal npcs for rng bundles (usually contain better manastones than you gave), or sell them on broker for people doing the same.
Endgame you can either join sell runs (selling loot if you get anything interesting) or disenchant fabled or eternal grade gear and sell the enchantment stones. If you do not care about AP you can also join Esoterrace runs from lvl 50 and onwards where you only loot the first boss and open a chest for a buyer (just take care and don't loot the contents of the box). Eso 1st boss loot runs usually go for 1.5-2kk.
