I've played a few stages of Azure Striker Gunvolt, and it's fast and challenging. Gunvolt can do the standard jump and shoot, but when he shoots an enemy it tags it making it more susceptible to his electric attacks. His electrical field doesn't prevent him from taking damage, but can destroy missiles that are headed his way while heavily damaging any tagged foes nearby. This field is powerful, but balanced out by the fact it only has a limited usage before it has to recharge. It can power back up gradually overtime or players can double tap the down arrow on the directional pad for a quick charge.
Gunvolt also acquires skills as he progresses, the first of which is an area attack. Skills have cooldowns before reuse, but it's pretty quick making it useful, yet not overpowered. As for other aspects of the game, checkpoints have a good spacing so far and health packs are not too scarce, which is helpful. The story in Gunvolt is kinda silly, but the localization is surprisingly impressive. The text is well-written and the goofy lines are actually more entertaining than I imagined they would be. I look forward to seeing where this one goes.
I was able to check out the free bonus game, Mighty Gunvolt, as well. It's a 2D platformer with a serious retro look which stars Gunvolt, Beck (from Mighty No. 9), and Ekoro (from Gal☆Gun by Inti Creates). Players get to select one of these three characters, each of which have different abilities to help them through the game's five stages. Gunvolt has a double jump and can do a charge attack that creates a beam in front of him that players can up and down. Beck's charge attack causes him to dash forward smashing things, and he also has a slide dash to get through narrow pathways. Ekoro has a basic charge attack that does more damage and can jump and float to cross gaps. This game offers the same classic 2D platforming from the olden days and is a nice bonus, even if you'll likely work through it pretty quickly.