Well, if you actually want to play as Genis, you'll probably be subbing Kratos and Colette in and out, depending upon the battle before you. Unless you have EX Skills traded over and you have Concentrate on Raine, Genis, or Kratos (I run it on all three of them. <_<) then you must have Lloyd on your team to prevent enemies from easily ganging you and preventing spellcasting. Colette attacks too slow and staggers too easily to manage more than one, two people at most.
Here's what I use: (in no particular order)
Colette: - Attack Close -Retain / At Once (At once typically for bosses, and I use Retain to cut down on usage of Orange Gels) - Don't Pursue
Lloyd: - Protect Friend - Save / At Once (see Colette's explanation) - Frontlines
Kratos: - Protect Friend - Save / Heal - Don't Pursue / Skills/Magic (if you're up against a boss, I turn Skills on so he focuses on healing)
Raine: - Reduce - Heal - Long Range
Genis: - Scatter - Save / At Once - Long Range
This is only covering up until you get Sheena, where things get much easier because of a reliable melee fighter not named Lloyd.
Of course, this is also going off the notion that I'm playing as Lloyd, so I can manage having this many people with spellcaster behavior. Making Lloyd a computer character is a bit of a gamble, but it takes guts, and I like that. I'm not sure how much help this will be, but I did my best. Also, if what I said is wrong, please, someone correct me. I don't want to be giving false information.
Get Rhythm, use Eternal (EX Skill 3), kiss your thumb good bye. If you feel lucky, use Randomizer (I'm fairly lucky with it. I generally end up with 3-4 insta-cast spells in Boss battles)
Go T route (I think that's it...whichever ended up with massive hybrid spells. I don't know why, but those seem to be some of the best spells IMO (Absolute = Awesome. Freeze Lancer fairly useful, Flame Lance is great). Those work very well.
I hate Inuyasha (I can't change my account name) Bring Lord Sturm back!
From what I've read you can use the EX skills tough and rhythm to get randomiser then you cast and cancel so that you will get instant casts more often. I would then use the tech glitch to get mid S spells and high T spells. Hope this helps.
If your planning on being genis, i suggest equiping EX skill, like Spell Save, and speed cast for sure. the main spell you should cast is iether tidal wave or meteor storm. if you happen to be in overlimit, cast indignation to use indignation judgement.