I do not think there is a assist command yet, but I think they are planning to add one.
In the mean time, there are several things that could help you regardless of if you play with a controller/gamepad or keyboard. I play with controller, so I am more prepared to help you with those options, let us know what you are using for more specific recommendations.
Text Commmands (Marcros): http://lodestone.finalfantasyxiv.com/pl/guide/macro01.html
Control and Keyboard Basics: http://lodestone.finalfantasyxiv.com/pl/guide/basis01.html
For me, using controller. I have two preferred ways depending on if I am playing a DoW (Melee) or DoM caster or if we are pulling groups or targeting 1 monster.
If melee and on controller, I will change targeting type to "Enemy Only" which changes your target recital to a red color and you will only cycle through monsters, any and all enemy monsters (regardless of claim status). Using my controller, the command is auto-run+confirm (R2+X in my case). To change back to all targets, R2+Square, Friendly only, R2+O, All targets: R2+Triangle.
The other option I use often is /btmode (battle targeting mode). This will change your targeting so that hitting left or right menu select buttons (up/down/left/right pad on gamepad on my controller) will target the Red "claimed" enemies, hitting up or down will cycle through my party members. Using this option as a magic class, enables me to target the enemy my tank pulled since I want to cast without getting hate, yet heal my party members without "un-targeting" the monster. I mainly use this when playing DoM "magic" classes personally, but it can be useful on single target fights as a melee too, like on Ifrit.
The game also has other targeting options that can be used in macros, see links above, such as <stnpc> (select non-player character) for a "nuke" macro, or <stpc> (select player-character) for a cure macro, etc. These will allow you to bring up a sub target selection option when using a macro, but keep your main target locked on whatever your current selection is. For example, target tank for cure and health monitoring, but use /ac "Slow" <stnpc> in a macro to be able to choose a enemy to cast slow on without dropping tank as your main target. There are more, but I do not use these too much due my play style and using controller.