You should try your very best to stick to one elemental theme in you rotation to build an astral (fire) or umbral (ice) effect. Ultimately, on bosses, you are going to find that Lethargy just doesnt work. That is ok. Regular enemies start with aoe sleep and lethargy. I use the following on bosses:
1. Thunder II
2. Fire III (Gives a start of Astral Fire III, would ordinarily take 3 Fire I to build)
3. Fire 1 (can trigger Firestarter which is a free instacast Fire III)
Note: I sometimes glitch the system by starting into Fire I when I have an active Firestarter. Then when Fire I is beginning the cast animation I will repidly press the fire III button and my character will cast Fire I for 500-700 and then immediately cast the Fire III for 600-1k. Ordinarily if you let the Fire I cast, then your Firestarter will go away, and if you stop and interupt your cast to get the firestarter fire 3, then you cant cast the fire I. This way I get both. Nice dmg spike.
4. Keep casting Fire I until you run out of mana (casting firestarters as you ge them)
6. Thunder II
7. Blizzard I until Transpose is active.
8. Transpose (will give an astral fire I effect)
9. Repeat step 2-8.
Hope this helps.
This rotation works until about 40 when you get access to the 3rd stack of Umbral/Astral. And yes I know Firestarter comes at 44 :).
Here is my rotation at 50.
Start with Thunder 2 - Its faster casting than Thunder 3. You can also start with Swiftcast -> Thunder 3 but it depends on whether or not you will need Swiftcast soon.
Start Fire I spam - I know you can go straight into Fire III but I don't for two reasons. First you can pull hate that early if you crit and the tank has not started their rotation properly and 2nd because Fire III costs a lot of MP when cast without Umbral 3 (more on this below) and if you can get Firestarter to proc its free.
I continue Fire I spam until I get sub 1000 MP, then I cast Blizzard III. This will immediately take you to 3 stacks of Umbral Ice and since you have 3 stacks of Astral Fire the MP cost and cast time is severely reduced.
Next I refresh my Thunder 2 dot and then cast Fire III. Again since you have 3 stacks of Umbral Ice the cost and cast time of Fire III is reduced and it puts you at 3 stacks of Astral Fire.
Now you can resume Fire I spam and repeat the process as above. Be sure to interweave any Firestarter or Thundercloud procs.
I only use transpose between packs if I need to refresh my MP or if I misjudge my remaining MP and cannot cast Blizzard III. And you can always have Convert to use in a pinch.