Here’s my recommendation:
Vanguards. These are units you deploy first to temporarily hold off the enemies while generating deployment points at the same time to let you deploy your main operators.
- Courier and Texas are great choices.
- Myrtle is a very good unit. You don’t have her now but do build her once you get her. She won’t reliably hold off enemies but she generates tons of DP. It may take you some time to get a grip on how to use her properly.
ST Snipers. Bring two of them. Their main job is to deal with flying units. Their secondary job is to help with the team’s DPS.
- Kroos and GreyThroat are very solid options.
- Level up Jessica to E1 at least. She plays an important role in your base (Reception Room, Clue 3 Search), and such role will only be available after promoting her to E1. Also, there will be maps where you will need more than 2 snipers.
AoE Snipers. They are used to deal with mobs, and building just 1 unit will suffice. They are ineffecient, however, when dealing with single targets only. In short, use them against Zerg Rush but not against Protos units.
- Shirayuki is the go-to unit for this role. You can get her via Recruitment using the following tags: Sniper+AoE, Sniper+Slow (Check out GP’s Recruitment Guide for more details).
- Unlike ST Snipers, AoE snipers do not prioritize flying units.
Casters. They are one of your main DPS source. Their primary role is to take care of heavy-armored units since they ignore their target’s DEF.
- Amiya is sufficient for dealing single targets.
- Mobs with high armor are best dealt using an AoE Caster. Gitano is a good choice for this.
Medics. Keeps your units alive. There are two types of Medics: ST Healer (single target burst healing) and AoE Healer (maintenance healing).
- Use ST Healer to keep a specific unit alive, typically your frontliner (e.g., your Vanguards). Shining excels on this role.
- Use AoE Healer to simultaneously heal 3 units with minor damages. Perfumer will suffice for this role; however, Ptilosis is currently available in the Shop, which is the typical go-to Medic.
- Strictly speaking, the role of ST Healer and AoE Healer is not interchangeable. However, for most of the current content, you can swap them just fine.
- Interestingly, it’s not their healing capabilities that most people are interested with when selecting Medics. They look on the Medics extra utility instead. This is the reason why Ptilosis (Global SP-regen Buff), Warfarin (ATK Buff) and Silence (Mobile Burst Healing) are usually preferred over Shining who mainly excels in just keeping a unit alive. Nonetheless, you won’t go wrong in investing on Shining.
Defenders. Ideally, your Vanguards, with the help of Medics, should be able to keep the enemies at bay while your Snipers and Casters do away with them. However, there will be enemies with high enough damage that will overwhelm your Medics’ healing capacity, resulting to the demise of your Vanguards and the collapse of your defensive line. In situations like this, you need to sub your Vanguards with Defenders. There are two types of Defenders, and you need to build 1 unit each.
- Pure Tank. They are the champions of the face slapping tournament. Enemy slaps them with a cub or a war hammer and they just shrugs it off. Coura, the Baseball Turtle, is the golden standard for this role. Use her as your main tank instead of Liskarm. Liskarm is great but for a different reason. For pure tank, vote for Coura and “Make Defenders Great Again!”
- Healing Defenders. Pure Tanks are great and all because of their high DEF. But what if enemies have casters too? Enter the Healing Defenders with innate RES! Well, actually their RES is lame, but hey, at least they mitigate enemy caster’s damage by 10%! And guess what, they heal their self too (as well as their nearby allies)! And since they are tanky (not as tanky as pure tank of course), they can take more hits than your Vanguards and thus a good replacement as well. Gummy on S1 works fine for this role.
- Defenders blocks 3 enemies (Vanguards blocks only 2) which means that they are very effective in holding off enemies at bay while your Snipers and Casters do their job.
Guards. You can pretty much live without them for most situations. But picture this: You already have deployed you Snipers, Casters and even subbed your Vanguards with Defenders yet enemies are slipping your defensive line, why? This means your DPS is insufficient and you can’t kill the enemies fast enough before they overwhelm your Defenders’ blocking capacity. This is where Guards come in, your other main source of DPS.
- There are a lot of types of Guards, but their main role is to supply the extra DPS your team needs.
- There are Guards that supply Physical DPS. There are also those that supply Arts DPS. You may want to build at least one for each damage type.
- Guards also come in different ATK Ranges as well as Block Counts. You might need to build one for each type as well.
- Due to this diversity, it’s difficult to build one unit per type during early levels. It is recommended to build them only when you need them.
- You can use Guards, instead of Defenders, to sub your Vanguards. Note, however, that Guards are not as tanky as Defenders (although they are much tankier than Vanguards), so they might get killed by hard-hitting enemies. As Lao Tzu said, “know your [enemies’ capabilities] and know your [own limits], and you shall be unbeatable in hundred battles.”
- Specter is a very safe choice to invest on. She’s an AoE Guard (hits 2 enemies at the same time; 3 enemies at E2) and thus deal very good amount of damage. At the same time, she is one of the key units in advanced gameplays (due to her “invincibility” skill).
- Mousse is an acceptable choice for Arts Guard, just in case you need extra Arts DPS.
Nukers. Your secret weapon to kill a boss or annihilate a very huge crowd!
- Requires E2 Promotion. (I can’t think of one who doesn’t need to be)
- Just borrow them from Friends; it is easier that way.
- Angelina can do this job for you.
Tactical Units. These are units you build outside of your typical “tower defense” mode.
- Assassins. Melantha and Vigna can be used to eliminate enemy Elite Casters and other key enemy units (e.g., a radio guy who will expose your hiding units).
- Bait-and-Stall. Let Gravel bait the bombs from those mosquito drones or let her take some hits from those axe guys to stall them if your defensive line is not yet ready.
- Derailers. Use Shaw and CliffHeart to derail enemies path, pushing/pulling them toward the pit of doom.
This would be how my squad would look like if I were you. I don’t have GreyThroat, so I put Meteor on her place instead. Also, disregard the levels you are seeing. I already cleared all of the current contents, and my main squad are on E1L50 and above. Some of the operators displayed are not leveled because I have better units than them. In your case, it would suffice to have all of them at E1L20.
Depending on the map, you may want to replace Gitano with Shirayuki or with Specter. Also, if you don’t need two Defenders, you can bring Angelina in. Or if the secondary sniper is not needed, bring Vigna instead. All in all, I recommend you focus on those units in the mean time. Aak is a very fun unit (stick to his S1 for now), but try not to use him first until you get the hang of each roles.
I might've forgotten some things, but I hope these can help you somehow. As always, things from the internet are bland, so take it with a pinch of salt.