Okay, so, good news bad news.
Good news: I statted up my summons. Well, tried to; they don't have CON or weapon profs listed, so I just assumed they're not slowed by their weapons.
Bad news:
Soldier Battle Stats (Steel Lance):
AT: 22
Hit: 94
AS: 10
Eva: 20
Crt: 6
DG: 0
Gargoyle Battle Stats (Steel Javelin):
AT: 22
Hit: 91
AS: 12
Eva: 29
Crt: 6
DG: 5
They can deal 10 damage to the druids half of the time, meaning they'll need a little under 8 turns to kill one on average. Throw in transportation time, especially if one dies and needs to be resummoned, and that is clearly more than the time it'll take for them to just run out of charges.
So, they can still harass them, certainly, but they're not going to be siegebreakers unless we're already outside their range.
In related news, I cannot tell what anything on the map is, but they have the following stats against the following enemies:
Sentinel: 36% chance of 3 dmg
Elite Guard: lolno
Elite Axeman: 35% chance of 7 dmg (-weapon triangle)
Swordmaster: 43% chance of 9 dmg (+weapon triangle)
Sergeant: 63% chance of 6 dmg (+weapon triangle)
Bishop: 40% chance of 9 dmg
Druid: 55% chance of 10 dmg
Wind Mage: 48% chance of 10 dmg
So clearly not great, but as wallphasing respawning cannon fodder goes, not something to discount. Unfortunately, while nothing but the druids can one-shot them without a crit (30 HP, 10 DEF, 9 RES), they only have 12 AS, meaning everything can double them and thus one-shot them after all. Their Evasion is also only 29, so they can't really depend on miracle dodges to avoid it.
All that being said, if Riven's not attacking she has nothing better to do, meaning she's basically a crappy- but unlimited- siege unit in her own right.
Oh, but the immediate point of all this is that I don't think poking the druids is the most effective thing to do right now. More likely we'll want them walling off the southern group or saving a real damage dealer their turn by poking the west one.