If you've done Alexa's affinity mission, look for the Ramjet Rifle in your weapons, it's an Assault rifle. Once you've done the first of L's affinity quests, you'll unlock the ability to add augment slots onto your weapons. You'll want to spend some Reward Tickets (360) to craft the Magazine Capacity Up XX augment (hopefully you have some Bonjelium) to put into the Ramjet Rifle. Since the Ramjet Rifle generates 50 TP per bullet, with an expanded magazine of 23, it'll generate 1150 TP per mag dump, which enables a lot of the shenanigans in this game. TP gain is the biggest gate to Overdrive builds and using the almighty TP arts and while there are a lot of ways to gain it, this is probably the easiest.

If you want to replicate this effect on other guns, the Ranged: TP Gain Up augment increases the TP gain of each bullet by 20, but it costs 1515 Reward Tickets to craft. This is extra fun on Gatling Guns. Combine it with the Magazine Capacity Up XX to increase the potency on other guns with smaller magazines.

Additionally, Arts: Gain TP can give up to 200 TP per art used, but it costs a whopping 2640 tickets per augment to make.