The viable battlegroups fall into three categories:
1. Faction (meta) battlegroups. These are constructed from one of the following 8 keywords: Grassland, Woodland, Underworld, Highlands, Marsh, Desert, Ocean, Crypts, or Tundra.
Faction battlegroups give access to a wide range of spells, relics and equipment and let you pick the best champions from several species.
In the current pre-alpha the viable factions are Grassland and Woodland. For all other faction keywords it makes more sense to use a species (see below).
2. Species battlegroups. These pick units from a single species (e.g. Minotaur, Fairy etc) and usually from within a single faction.
Species battlegroups can access their faction's spells, relics and equipment and also have access to a banner relic and other unique spells, relics and equipment. Species tend to have a strong defining "mechanic" and are more complex to play than a meta battlegroup that just has strong individual champions. Each current species is described in detail in the next section.
3. Themed battlegroups. These pick units that share one of the many keywords that are neither a faction nor a species. This will have greatly reduced access to spells, relics and equipment but may include a few unique choices. Examples of currently viable (tier-2) themed battlegroups are: Soldier, Beast, and Fire. More probably exist, and more will emerge as new runes are added.
Strength: Tier 1
Banner: After activating a Leap, its cooldown is set to zero.
Anurine have access to many spells and abilities that produce mud and water (impeding terrain). Some of these are tied to Leaps, e.g. Tidal Leap which are improved by deploying the banner. Multiple Leaps also let Anurine champions ignore the penalties for the impeding terrain they produce.
Anurine have a second core mechanic of evolution. Anurine Keepers can cause Tads to grow into Pupils. Anurine Pupils can grow into Spitters or Warriors after three turns. Most importantly, the Anurine Mentor has the Evolve Anurine activated ability. When this is used on an Anurine unit it finds another friendly Anurine unit that costs no more than 10 essence more, and the target evolves into a copy of it. This provides a good scaling of value over time, can be used to adapt from ranged to melee (or back), and works around the usual 2 copies per battlegroup limit.
Strength: Tier 1-2
Banner: When opposing spells are cast all friendly champions gain a rank of Swift (refund 1 AP after moving) for 1 turn.
Unique Spell: Wild Stampede. All centaur gain Trample for 1 turn.
Centaur champions are designed around movement. They tend to have high speed but also Ponderous, limiting their attacks to the normal rate. The "combat" Centaur come with abilities like Blitz and Running Strike that increase damage and range as they move.
The second type of Centaur champion is the "musician". These each come with an activated ability that triggers on each move on the turn it has been activated. Each trigger causes a buff to friendly musicians nearby (such as granting Mobility, Empathy, or a limited Cleanse), and also a debuff around the friendly musicians (such as Distracted, Charmed). Finding efficient movement paths to get maximum benefit from these abilities and the Wild Stampede spell gives room for higher level play.
Strength: Tier 2
Banner: None (yet)
The Dwellers have incredibly powerful ranged units with incredibly significant drawbacks. Each siege Construct champion has the Contestable ability. This means they can be contested like a Font from within 3 spaces, in addition if they become neutral they will become engaged with your own units! It also means they cannot be used to contest Fonts.
The game plan for Dwellers is to use your non-Constructs to prevent the enemy approaching your siege engines. Several abilities help with this: Secure Allies (negates Contestable for two turns), an aura granting Unmovable (immunity to knockback), an aura granting +1 range for the Constructs, Latch On.
Dwellers also excel in defence stacking.
Strength: Tier 1-2
Banner: Has Adaptive: Casting and permanently grants Adaptive: Casting to the Shine. This increases spell presence by 1 when friendly spells are cast.
Fairies have a mixture of very fragile, ranged hovering champions, and more tanky corgis. All hovering Fairies have Gossamer Wings that are removed when they reach 15HP, at which point they also become Defenceless. Individual champions can cause Charmed, Passive, and Illuminated. The Unicorgi grants rare access to Dispel (and Cleanse). There is no explicit mechanic for synergy for Fairies and so they have low complexity compared to other species.
The Mushroom Ring relic is available to any Woodland battlegroup, but grants stat bonuses for Fairies only.
Strength: Unclear (God Tier - Tier 2)
Banner: Reveal two champion runes. Grants Acid Eater to all champions.
Unique relics: many
The four (two copies of two) Gelatinous champions provide a full battlegroup worth of HP. This is the most efficient essence-to-HP ratio by far. However in their basic form they are fairly useless: no defence, no attacks, and low movement. Each champion has the ability to Divide or Recombine that lets you manage the number of units and distribution of HP over time.
The use of relics is key. The unique relics grant abilities to nearby deploys (including deploys via Divide). Most of these grant both a positive and negative abilities, e.g. Essence Miner + Lumbering; Immunity Frost + Immunity Electric + Resistance Acid (reducing Acid Eater heals); Arrow Eater + Defenceless etc. One relic also grants Swallow Whole - though this can only target units with less MaxHP than the attacker.
Overall it should be possible to slowly customise the champions to counter opponent BGs. This battlegroup may be susceptible to rushing.
Strength: Tier 1
Banner: Opponent units return to play 1 turn late (return to play includes Teleport, Dimension Door use, returning from chasms, Vertical Push, Escape Magic etc.)
Minotaurs have very durable and hard hitting melee units. Several also come with the Bullrush ability that can help close quickly, and Minotaurs can be played effectively without any other special tricks. However careful use of their Dimension Door relics should greatly improve the battlgroup.
Dimension Doors (including the banner, and summoned by Gatekeeper) can be teleported between when they are in-line up to 10 tiles distant. Moving next to a Dimension Door triggers its ability, if another door (or any Portal Exit, including one Minotaur champion) is inline on the far side of the door the champion will relocate to the far side. Understanding this mechanic is vital to making use of Dimension Doors.
Higher play may also include setting up Doors to exit into chasms, sending opponents through using knockback. Chaining doors together to maximise Portal Walker procs etc.
Strength: Tier 1-2
Banner: Every deploy adds a 10% discount to all Ratta (up to 60% maximum discount) for 6 turns.
Rattakatta has a mixture of expensive, individually strong units which also provide stat buffs like Battlemaster (the Katta units) and individually weak units that are incredibly cheap and that can be deployed en-mass (the Ratta units). Maximum value comes from having many Ratta units buffed by a few Katta and this is made explicit with the Unruly Rat ability that debuffs any Ratta too far from a Katta unit at the beginning of the turn.
Careful use of the banner can increase the efficiency of the battlegroup -- saving up essence to dump a few relics and Katta before a mass deploy of discounted Ratta. This is still relatively straight-forward compared to other species' mechanics.
The Ratta units also have a Disease sub-module, which is the "medium" complexity part of this battlegroup. Diseases are negative conditions reducing stats over time that can be passed on to other units. Two Ratta units have Carrier:<disease> abilities that grant them that disease, and immunity to it, as well as the Rabid ability that means they pass the disease on when they cause melee damage. The very cheap Mad Ratta champion also has Rabid (permitting it to pass diseases) but does not have any immunities. With very limited access to cleanse, disease should be an effective resource for longer games.
Strength: Tier 2-Fun
Unique Relic: Pure Cheese
Ratta (without Katta) are crippled by Unruly Rat but come with what would otherwise be completely overpowered units. The King Rat (only available to Ratta battlegroups) is a mobile deployment zone that can grant Blocking to all nearby champions. The Pure Cheese relic lets Ratta champions multiply their attacks: they all receive Pack Member and Leading the Pack which means that when one Ratta attacks, all others adjacent to the target attack too; and it grants Sentinel, multiplying attacks on the opponent's turn.
Even with the special extras, Ratta are not really competitive. If you want to run them without Katta they can also be run as a Grassland battlegroup with the Pied Piper (Centaur) and/or with Katta Scarecrow (relic) to work around Unruly Rat.