I thought we could have a discussion / brainstorm session on practical tactics and combos with the summoner class.
I'll start first.
I just finished building my first playtest character here, and have written out what I believe to be some basic tactics such a character could use.
Direct link to sample summoner
Since there is no duration for summoning an eidolon, unless it is socially unacceptable to have a terrifying beast following you around, have Salamander already called forth at the start of any encounter. Similarly, you should always have your longspear at the ready, since you can't exactly tuck it away in your pocket.
If available, open up combat by casting evolution surge to make Salamander huge size. Unless you have multiple encounters in rapid succession, you should be able to do this in most encounters, since it is a focus spell. This will grant him substantial reach, which can be used in conjunction with movement to force enemies to waste actions closing. It will also likely make him the focus of attention while you and the party healer in the back row can help keep him in the fight by healing you. Use your wand of haste to give yourself and your eidolon an extra action, resorting to your spell slots to do the same in later encounters only after you've exhausted your wand. Use dispel magic against particularly troublesome spell effects or magical items, or fly against hard-to-reach targets. Otherwise, conserve your limited spell slots for haste and similar buffs so you can better extend the effectiveness of your adventuring day.
Durin's attack spells, being cantrips, aren't the most damaging option you can use, but work well against enemies that are out of reach of you and your eidolon, who have elemental weaknesses, or when Salamander uses Primal Roar to debuff their AC and saves first to increase the chances of a getting critical success (also useful if you have other casters in the party with more potent spells).
Unless the situation demands something different, keep Durin out of harm's way, defaulting to long range cantrips like ray of frost or, if forced in close with multiple threats, use multi-target spells like electric arc. There's rarely going to be a benefit to having both of you getting attacked at the same time, or having to use the lower of your two saves against area spells.
If Salamander has difficulty hitting a target, and no other allies are nearby, it may be worthwhile to use abilities like Tandem Move to position into flanking. Using boost eidolon to ensure the hits that do land do the most damage possible is also a good idea. When fighting against multiple enemies, using reinforce eidolon might be better for increasing your staying power.
A typical round after evolution surge and haste might be as follows:
1) Tandem Move to get Durin to reposition as needed (usually to get to cover or some other safe place, or to flank) and Salamander into melee reach of his enemies.
2) Act Together to get Durin to cast boost eidolon and for Salamander to make a Strike from reach. (Alternatively, don't cast boost eidolon or use Act Together so you can later cast a cantrip in 3b.)
3a) Salamander delivers a second strike.
3b + 4) Durin casts a saving throw cantrip, such as electric arc to avoid multiple attack penalties.
4a) Salamander moves back, forcing melee enemies to waste valuable actions chasing him. (If Durin is also nearby, use Tandem Move to keep him back as well.)
4b) If Salamander can't distance himself (such as when fighting in small spaces), consider casting reinforce eidolon instead so he can weather the storm, or use Primal Roar to debuff the enemies' attacks and make them weaker against your allies' attacks and spells.
If things go south, send Puddlejumper away, and have him return with help or to rescue you later. He's not only great for fast overland travel, he's a smart, well-trained horse capable of doing quite a lot for you out of combat. He's fragile though, so err on the side of caution. Try not to get him killed, as he represents a substantial downtime investment.
Combat maneuvers are very effective to be used on enemies, specially because it can reach to legendary and the Eidolon take your item bonus from it as well as having high strength.
Let's suppose that enemy disengaged the Eidolon and came to attack the summoner, you could per example Stride the Eidolon to the enemy, then act together for the Summoner to Aid and the Eidolon to trip or grapple the enemy, immediately using the reaction to give a good amount of circumstance bonus and then with the last action make the summoner step or stride away.
If the Eidolon is already engaged, a demoralize + trip and then a spell/cantrip can be nice too.
act together (boost+move) attack attack
move in tandem (move+move), act together (boost+attack+, attack
enemy is flying, evolution surge for flight, same for swim.
possibly first turn actually cast something like heroism on some other martial in the party.
Let's suppose that enemy disengaged the Eidolon and came to attack the summoner, you could per example Stride the Eidolon to the enemy, then act together for the Summoner to Aid and the Eidolon to trip or grapple the enemy, immediately using the reaction to give a good amount of circumstance bonus and then with the last action make the summoner step or stride away.
Ooh I forgot that Aid was totally an option!
If you have Tandem Move, you can use that first action to get the summoner and the eidolon into flanking as well, further increasing your chances of success.