For Freya one of the easiest things is to set up some targetting macros /tar Storm or /tar Ancient or /tar Snap - I'm not sure how your guild does the fight but I'm guessing you're never needed on one of the adds. Targetting macros for the trees that grow would also be useful.
Another thing that can help is do you use oRA2 (or some other addon) that allows Maintanks to be set, you can then see their target (square health bars for both tanks and tanks targets - size can be adjusted, not sure on colour), this would enable you to click on a particular tank target to target it rather than searching for it in the mess!
Another option, whicjh wouldnt help with freya) but would on some other fights is setting up focus macros, and showing your focus on your UI, allowing easy change back to a specific target.
There are nameplate addons like, Aloft, which allow you to change the size of the healthplates above monsters, also to increase the size of raid marks on them. have a look in the user interface forum, you may find a UI with some addons that you think might help.
Best of luck and persevere with it! I used to tank with a partially deaf tank, and found it was mainly a matter of setting up his UI very well and the rest of the raid knowing about it. Yelling at him in vent for example was pretty useless if there was other chatter going on, so the other tanks set up "/y move", "/y taunt focus" macros and it worked really well.