Yes, the Irabaar lilly is irritating, and there's another one in a battle (quest with girl gone to study with a druid, the druid is in form of wood troll and controlled by the lilly). I needed help too with this one (two). I gathered tips by Google searching and won both battles, with some difficulties. Here are some tips:

– attack only the lilly, ignore wisps, tentacles, troll, concentrate to kill the lilly – as soon as you do, the battle ends
– have your melee fighters stand side by side so that they are not completely surrounded by many tentacles (this can be tricky though since the tentacles can knock them down easier and you'll lose turn, but in this battle as I recall you'll get surrounded by four or more tentacles anyway, so it's better to minimaze the count of tentacles attacking)
– Hammer Blow skill is always great thing to have
– if you have bows and crossbows (I trained Zurbaran and Aurelia in ranged), use them, they are very effective
– shoot from afar, try to stay on solid ground where tentacles can't attack you
– Triple Shot skill (bow only) is real killer
– don't use offensive spells, healing and support spells (Balm of Healing, level 3 Move as Lightning, Standfast Catlike etc.) are much more effective (this especially applies to Blackguards in general)
– don't mind the character control, only one is controlled at the time, doesn't do any harm, doesn't get attacked, and can be pulled back or resume melee attacking when the effect wears off
– do other quests first, if you have left any, to train some more

My main character is a melee fighter. When I had one of my magic users controlled I just restarted right there. I don't think stacking the melee fighters works, because the tentacles will block line of sight, and you won't be able to heal them. What worked for me is I positioned my 2 uncontrolled melee fighters on 2 sides of the lilly, as far away from each other as possible. I then could attack it with magic (fireball, and controll elements). One of my melee fighters was knocked down as expected, and managed to hit the lilly with a hammer blow for 45 with Naurim. Apart from getting lucky, and trying over and over again, I just don't know what else would work. Apart maybe from getting high double vision and casting it on melee characters so they avoid getting hit by the tentacles maybe. Still the double vision on the lilly is quite annoying.

Edit: I noticed that ranged attacks deal quite nice damage, but Aurelia and Zubaran weren't that invested in ranged attack at the time. Maybe on a second walkthrough on hard mode (if I have the nerves for it :)).

Yes, double vision 3 is a very nice spell to have for these fights. The speed spells are good too, for getting faster into melee range.

That said, I didn't have your problem : My main char is an archer, and the flowers became really easy as soon as I got triple shot.

(In fact, an archer is nearly useless in some of the maps, but many others become cakewalk

Aside from the things that where allready mentioned
-Klarum Purum IV will give your Party a Poisonressistance for some turn but buffs might be more usefull
-a high Evasion will help as allways
-you can block spaces next to your planned attackposition with the spell Fortifex to avoid some spawns
-Paralysis IV is quite usefull too
-Kulination IV will create a stormcloud for several turns dealing damage each turn to eveything within

And you can counter the Lilies-control with magica infracta...but at the beginning of each turn where no one is under "her" control she get a free use of "her" controlability.

On my first fight I missed the damn thing 10 times in a row with my ranged characters (all "60%" chance). A bit later when things gone bad (had 2 characters dead at the time) I hit the lilly 5 times in a row. Maybe the RNG gods want more offerings I guess...

Great idea with the Fortifex spell, good thinking!

Oh..and i forgot to add...getting one of your chars controlled is usefull most the time because he will be moved next to the lilie without being attacked, used to your favor togather with the fortifex you can get your chars to a attackposition without spawning a single tentacle.