Here is a Hybrid Bossing Build suitable for 90% of content:
1st Job (Mage) – 120 SP
2nd Job (Summoner) – 230 SP
Total: 350/350 SP
Follow this strict leveling order to avoid respeccing:
Respec Note: Dragon Saga gives a free skill reset at level 40. Use it to transition from PvE farming to bossing build.
Pro Tip: Do not summon Ice Elemental for bosses. It slows the boss, which sounds good, but it also slows the boss's animation frames, sometimes delaying dangerous attacks unpredictably. Fire is safer.