It's the best non-specialized sustain duo in the game (during prolonged battles), and will work relatively well with a tonne of other duos. If you want a generic non-specialized backbone to help you get started through the worst of it (without too much spoilery info), then a combination of VES+JES by itself will cover many bases. There are countless comps that can dunk the basics (Miller > Sleeper > Enter Endless) - and to go in-depth might spoil it for you. Keep what you learn in mind for the future.

You will have to face him again in Endless Mode, so Veteran is basically a tutorial. Just like most bosses, the Miller is not that complex, but he does involve various gimmicks to learn and keep in mind. In this case - level 4's with Upgraded Weapons, Armor, and good (relevant) Trinkets. Just like any Veteran mission, be sure to use fully upgraded heroes. So, make sure you are adequately prepared. The Apprentice & Veteran tutorials can claim dead heroes for good, whereas the main endless mode will simply steal them temporarily.

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