My character, winged tiefling lore bard (10) and divine soul sorceress (2) has entered a tournament against other spell casters, and I need help for 1 on 1 strategies. 

STR: 8 | DEX: 16 | CON: 14 | INT: 12 | WIS: 12 | CHA: 20

She has the ability to true polymorph into creatures of CR 12 or less provided they have a swim speed. She also has access to level 6 spells because we homebrew spell casting when it comes to multi class so she has access to all bard, sorcerer, and cleric spells up to level 6. 

When it comes to fighting clerics, bards, and wizards I think it's pretty standard. I don't know much about druids so I don't know how painful they will be. I am mostly concerned about fighting sorcerers with meta magic. 

Any advice on how to beat spell casters in a 1v1 match, any advice for a good form to true polymorph into, and, most importantly, any advice on how to win out Counterspell duels and to circumvent meta magic, especially stuff like subtle spell. 

I've been toying with opening with greater invisibility or fear, but even though I am usually my party's blaster caster (our sorcerer is the support) I realize I don't have a lot of damaging spells. Any advice on which to pick? My go to is to usually true polymorph or use animate objects, but both of those are concentration, meaning I can't use greater invisibility or fear, which sound like they would be very useful in a 1v1. I could also use the slow spell. But I don't want to have too many debuffs in my arsenal and not enough damage, so if you could recommend some strong damaging spells and just the most reliable buffs/debuffs you can think of that would be grand. 

Young black dragon is cr7, with a swim speed. 
And young gold dragons are cr10, with a swim speed. Young amethyst is cr9. There may be others if you poke around.

Fly up, breathe, fly up more so you’re out of range of most spells, like 65-70 feet or so, circle until your breath recharges.swoop down until you are 30 feet away, breath, go back up. Repeat as necessary.

Hero’s feast is a great level 6 buff, no concentration, though it has a pricey material component, and you need time to prepare it.

Slow is not very useful against most casters, because they aren’t typically going to use multiple attacks (except polymorphed Druids), bonus action attack spells, or offensive reactions (cf Hellish Rebuke).  Contagion poisons for at least one round automatically and you can customize the disease against their best stat if you know the kind of caster you’re up against.  It’s likely to be a killer debuff until/unless they save.

Low level STR save spells could also be effective, since many casters dump STR.  Entangle and Web come to mind.  Get out of range of their closer spells and hit them from a distance or retreat to heal.  Both are concentration, so they would be for after your Greater invisibility is done.

Quote from Xalthu >>

Young black dragon is cr7, with a swim speed. 
And young gold dragons are cr10, with a swim speed. Young amethyst is cr9. There may be others if you poke around.

Fly up, breathe, fly up more so you’re out of range of most spells, like 65-70 feet or so, circle until your breath recharges.swoop down until you are 30 feet away, breath, go back up. Repeat as necessary.

Hero’s feast is a great level 6 buff, no concentration, though it has a pricey material component, and you need time to prepare it.

 These are good options. But you could go for a Yuan-ti Anathema for spells, magic resistance, and a potential 15d6+30 damage per turn and 189 hit points to bite through.

Quote from Sanvael >>

Quote from Xalthu >>

Young black dragon is cr7, with a swim speed. 
And young gold dragons are cr10, with a swim speed. Young amethyst is cr9. There may be others if you poke around.

Fly up, breathe, fly up more so you’re out of range of most spells, like 65-70 feet or so, circle until your breath recharges.swoop down until you are 30 feet away, breath, go back up. Repeat as necessary.

Hero’s feast is a great level 6 buff, no concentration, though it has a pricey material component, and you need time to prepare it.

 These are good options. But you could go for a Yuan-ti Anathema for spells, magic resistance, and a potential 15d6+30 damage per turn and 189 hit points to bite through.

 Anathema is definitely my favored shape but I'm trying to save it for a later fight (as in don't reveal that I can do that until later so people can't devise a strategy around it), however I have been really considering either young amethyst dragon for blink and the strength save breath, or even the adult deep dragon for the wisdom save breath and the legendary resistances. 

Subtle-spell metamagic will be your friend. 

It'll be my enemy. I don't have meta magic yet, but my opponent does. So I need a way to beat subtle-spell. 

Quote from millenialwriter >>

It'll be my enemy. I don't have meta magic yet, but my opponent does. So I need a way to beat subtle-spell. 

 
There isn't.

You're screwed.

All the best.

You can beat subtle spell. The whole purpose is to prevent the enemy's counterspells and keep your spellcasts safe. So, lets focus on ways to do that without using subtle. Ways to put you back on even footing with a subtlespell using counterspeller.

Counterspell has a few requirements. The enemy needs:

• A reaction available to them
• To be within 60ft of you
• Sees you casting

Interrupt any one of these 3 things and they're not able to counterspell you.

The easiest is to duck out of line of sight somehow, behind total cover, and cast from there. Now, you won't be very capable of attacking this way, for the obvious reason of also not being able to see them. Alternatively, just using a bit of movement to duck out of counterspell range is often the trick, since many spellcasters don't intentionally try to close the gap in the same way more melee types might. If you have the space available, add some distance between you and them, and keep it while you can. Other options are to simply prevent them from seeing you more directly: invisibly, darkness, fog, etc. If they can't see you they can't counterspell you, after all.

All this extra work does, unfortunately, only put you back on even footing with a subtle speller. You'll have to rely on your normally-unachievable spell diversity to win you the day, that and the 9th level spell you got access to. leverage these strengths and you'll do great.

Go Dracohydra. https://www.dndbeyond.com/monsters/dracohydra

Against a sorcerer, cast Anti-Magic Shell, then poke them with a dagger/rapier until they go down.  Can't use metamagic if you can't use magic.