My buddy mains Tager, and we've fought each other a lot.
He's given me some good tips.
Tager will rely on tried and true combos to pack on massive damage, so cancel out of the throw combos.
From the times I've played him, I find its not best to pressure Tager too much, but the 6D j.D pressure can come in handy sometimes, use it very sporadically, or you'll eat more throw combos and tager busters than you should. In general, keep a moderate distance between you and tager, and rely on your 5B and 5C to poke and start combos
Also, it never hurts to ID when you're up close, utilize it more against Tager and end it with 236C to wall bounce, as Tager is big enough to be caught in another ID string as he bounces off the wall before he can recover.
Be careful when magnetized, as it closes the distance between the two of you rather fast when Tager attacks, I know its caught me off guard plenty of times. However, it does allow you to counter from a further distance if you can time it right. I haven't messed too much with IB'ing yet, so I can't say much on that front. Avoid accidental bursts, and watch Tager's tension; I've been finished with more distortion drives than I can count because I wasn't paying attention and rushing forward to pressure Tager.
Overall, its not a bad matchup. When I first started out I thought it was just ridiculous how much damage he could put on Ragna in one combo, and magnetism didnt help either. However, I just had to play ragna smarter, and actually learn what connected into what. Wait patiently, choose your attacks carefully, and punish any missed moves.
Hope that helps.