Summoner has the shortest amount of casts. Gemshine is your strongest tool.
Your rotation at level 50 should be:
Summon Carby when needed (if he disappears)
-> spam carby skill
-> open up a summon (green, yellow, red lego, your choice)
-> spam *gemshine* skill (it'll change, during your "Titan" phase it'll be an instant cast on a 2.43 GCD, garuda it'll be instacast on a... shorter GCD i forget how long. And ifrit has the only casts youre going to have, 2 casts of ifrit's "gemshine" skill.)
This means you'll be casting your 2.5s skill roughly twice every 30-ish seconds. If your rotation doesn't look somewhat similar to that, youre doing something dreadfully wrong.
At later levels, particularly level 90 (I forget how it is at earlier levels XD, i've only been doing savage on smn lately)
Your rotation should look like this
Summon bahamut
Spam bahamut *instant cast* skill + Weave in ogcd Energy Drain/Fester
open up a summon (green, yellow, red lego)
spam summon "gemshine" skill + "astral flow" skill (ifrit has a 2-part melee combo, garuda has a hardcast DOT, titan has a secondary instant cast skill that procs immediately after using gemshine.)
Ifrit phase will look something like this: hardcast skill... hardcast skill... gapcloser+melee skill (both instant)
Garuda phase: instacast, instacast, instacast, instacast, instacast, instacast... hardcast dot
Titan Phase: instacast, secondary instacast, instacast, secondary instacast, instacast, secondary instacast, instacast, secondary instacast
Ruin 4 (instacast)
Youll notice there are only 3 hardcasts there. The rest of summoners rotation at level 90 is 100% instant casts unless you need to use a filler ruin