What are good ways to earn badges and level up in Ingress?

For reference, all the medals available in Ingress: https://ingress.fandom.com/wiki/Medal

You need at least 7 medals at gold rank or higher to be able to reach max level. The eight easiest medals to level to gold are:

• Recharger: recharge resonators with XM. XM is plentiful, and there's always rechargers to be refilled. Much easier if you're in an area with a lot of level 8 resonators and not a lot of opponents to destroy weak resonators.
• Translator: successfully complete perfect glyph hacks on portals to earn 6000 points. Each perfect hack earns you 1, 2, 4, 8 or 15 points, depending on how many glyphs there are. Glyph hacking takes a while, it's kind of hard to master, and you can't really do it at speed like you can with regular hacks, but it takes an order of magnitude fewer portals than the Hacker badge. Try and build up to it by practicing on lower-level portals until you can remember the name of the shape rather than having to remember each node.
• Sojourner: hack every day (don't go 24 hours between hacks) for at least 60 days. This takes two months of regular play (but then so does getting to the level where this is relevant), but many players have a nearby portal they regularly check into, and many players will play during their commute. You're likely already on the way to this through normal play.
• Trekker: walk 300km with Ingress open. You'll get this with your phone open while playing regularly. Be aware your phone has to be unlocked with Ingress open for this to count - it won't count if your phone is locked and in your pocket.
• Explorer: visit 2000 unique portals. This is much easier to get than the non-unique variant, and synergises well with Specops, below.
• Specops: complete 100 missions. This is a lot easier to get if you look for mission art missions, which tend to be designed in a sequence with an obvious path to follow - one set of 24 missions will get you nearly a quarter of the way to this medal. Cities that have held mission days often still have the missions around.
• Pioneer: capture 1000 unique portals. Synergises well with missions, which tend not to be well-guarded, but if your normal play area is held by the opposition, this is going to be much more difficult to get.
• Illuminator: capture 250,000 MUs with control fields you complete. This is a pain to get solo, but your local player org likely will run fielding ops. You'll be able to get big fields, and densely layered fields, this way, and fields that other players will try and defend until the checkpoint.

After level 13, you need one additional platinum medal per level, and to reach level 16, two of those platinum medals will need to become black medals. According to this Reddit post, the five most common black medals were, in order of rarity, Recharger, Translator, Trekker, Illuminator and Specops. Sojourner becomes increasingly difficult to preserve for the full year required to collect the black medal - and it's reset if you're too busy to play Ingress even a day over that year. The amount of portals needed for black Explorer and Pioneer is more than is probably available in your city.