Well, invoker is certainly not a simplest class in CF.
There are couple FAQs on Dios, tho imo they are a bit outdated.

Here are some advices:
1. Get yourself a tome. Powerrank to 38, then go for mastering your spells and getting lesser shields. Some vokers prefer to master the spells once they get them and then they rank further - this is a mistake. After you got them, master air shield (unless you are an arial) for shield of winds, watershield for shield of waves and fireshield for shield of flames. These three greater shields are most important to my experience.

2. Search for your sleek rods. Do not hesitate to ask all non-enemy mages and your cabalmates in some RP-ing way about this. Once you stumble upon me you can be almost sure you'll get your sleeks

3. Continue mastering spells and gearing yourself. Obviously, you have to gear for hp/svs/svp.

4. Quicksand and rain of stones are your lagging spells (I do not want to talk about pebble to boulder since it's kinda complicated), so try to get to them quickly. Shield of lightning can deal nasty damage so sometimes it will help you to seal the kills as well.

5. Learn about additional preps. ABS + 5 shields is often not enough against badass chars.

6. Learn your enemies so you know what greater shield (and when you become more experienced - which spells during combat) to put when you are going to engage.

If you wish, you can ask any questions to me privately.