If you take a look at playstyles of BLM/SMN, WHM/SCH, and PLD/WAR, you'll notice one is easy to play while the other is more complex to maximize then the other. That's where BLM and SMN draw the line, SMN is a complex DPS class, but all it takes is some getting used to. Go to Whitebrim in Coerthas and beat on the level 50 training dummies until you feel comfortable with the playstyle, but one important thing about playing a new instance is learning how to adapt to new scenarios you already know.

For Garuda HM I'd recommend Miasma 2 on the Plumes, try to use Garuda-egi's AoE on a pack (NOT THE ENKINDLE!), then stay on Garuda. Make good use of Place and Heel to keep your egi alive