I am getting irritated with Summoner in endgame content

[Ryuko]

Anyone who is judging overall DPS by looking at Turn 4 is silly. The phases are extremely diverse.

For SMN in Turn 4, here is what I do (summoners will vary):

Phase 1: Target shadowflare, follow tank in as he pulls, immediately cast shadowflare (not SC) as the bugs gather. DoT up the left-most mob, Bane. DoT up the right-most mob, Bane. Miasma II. Fester on something with high HP to waste your last Aethercharge, then Aetherflow so it is ready for the next phase. Blizzard II spam until most things are dead, then single target mobs with the most HP.
Phase 2: DoT up the first soldier, Contagion, SC Shadowflare, Bane (so no time is wasted). Switch to other soldier, DoT up... Bane. Nuke nuke nuke. DoT up again... Bane. Use Aetherflow to get your charges back for the next phase. Be sure to position Garuda near where the dreadnaught will spawn so you will have the spiders fed immediately.
Phase 3: RS + 3 DoTs, Fester. Miasma II, Contagion (while casting shadowflare... or SC+Shadowflare if your group is fast enough). Nuke nuke nuke... Fester, Energy Drain (while RS is still active). Aetherflow to replenish your charges. Rouse+Spur+Enkindle as this is the strictest dps race in the fight. Possibly use a few more Festers (if they are ready) before the phase is over. Don't forget 100% DoT uptime.
Phase 4: Follow tank that is taking the spiders (OT usually) and stand on top of them so they can AoE aggro them. Follow a similar strategy to Phase 1... DoT up one side, Bane. DoT up another side, Bane. Miasma II and Shadowflare for added damage, and Blizzard II spam for more AoE.

Note: This next part is where many groups differ. I'll be referring to how my group does it. We usually win right as the soft enrage hits.

Phase 5: DoT up the soldier, Contagion. Bane. SC + Shadowflare. Kill Soldier, switch to the dreadnaught. Nuke as usual until next phase...
Phase 6: This phase spawns another dreadnaught, a soldier + knight pair, a rook, and two spiders. As a caster with an instant cast (Ruin 2) I take it upon myself to grab at least one spider so that I can bring it straight to the tank. A bard usually grabs the other. The melee gets on the rook and limit breaks it (if we have limit break), while the rest of us finish it off. Use your usual dot rotation and Fester. Then, switch to the 'old' dreadnaught (should be between 50% and 75% HP by the time you start on it again). Go hard on this part -- RS if it's up, follow a similar rotation to what I said to do in phase 3 if you have the cooldowns for it. After the old dreadnaught is down, switch to the soldier + knight pair. DoT up the soldier and Bane as you did in phases 2+5. At this point, you have the option of finishing off the soldier and throwing off supplementary heals, or switching to the spiders and trying to down them so the end of the phase is easier. If your group has gotten to this point, it really doesn't matter. After everything but the spiders are dead, stack on the remaining dreadnaught's butt (remember to position garuda here too so she gets AoE heals) and do your full, hard dps rotation until the end of the phase.

Defensive cooldowns: Virus is really nice during phases that are really hard on the healers, such as when there are two dreadnaughts at once. Make sure the 'immunity' debuff is not on a dreadnaught when you use it. Eye for an Eye is also great for these moments... feel free to toss it on a tank.

Every summoner is different. I have found this to work for me, and my group. You'll find summoners nitpicking what I do, but don't let that distract from the general strategy of what every summoner should be doing.