Overview:
Many players are afraid of Fremennik isles because of final fighting part. Most guides even recommends to get high combat lvl (like 80+) before going to fighting part. But it is not really necessary. I did this quest four times so far on various combat lvls and last time I did it on lvl 38 combat (just to test following procedure) and it was still possible without big hassle. Fremennik isles is actually easier quest then Fremennik trials from combat point of view. It means that anyone who finished Fremennik trials fights on its own will be probably able to finish also Fremennik isles. The only additional requirement is to have lvl 20 defence or better. However, if you are not experienced low lvl adventurer, it might be better if you wait for 43+ prayer and like 40+ hitpoints before you go.

Background:
Troll king is strong and dangerous monster. It has very high defences against both range and magic. IF you want to kill him by distance attacks you will need to fire like 200 projectiles or spells. Even Iban blasts should be around 75 to kill this beast. Another thing making king really dangerous is his ability to change attack style according to used prayer protection and fighting distance. This means that he can always hit. But player is able to choose his attack style by setting "suitable" prayer protection. That is why brave varriors are using protection from range - it forces king to use ice barrage which is his worst and slowest attack. Never try to use melee prayer protection, it will make king really mad and you will be dead really fast. But with some superpotting before battle, unleashing of all dds specials and then finishing by whip, dlong or dscimmy king will die pretty easily. If you are brave warior able to face ice barrages and eventually a few 20+ hits by club...

Note that protection from range is essential for use of 2h weapon like halberd. Halberd is good option for players who do not want to get too close to king. Because halberd is twohanded weapon such player cannot use fremmennik round shield. And it is problem, because without this shield troll king can do up to 34hp damage per round by his range attack. And range attack is his default attack.

And now back to our cowards business. While without fremmy shield king can hit up to two 17th by his rocks, against this shield he can hit up to TWO hp only per rock. And now think about function of ring of recoil. Ring of recoil returns back 10% of damage rounded up to whole number. In case you are holding fremmy round shield and wearing ring of recoil, it will return 75% (in average) of damage inflicted by rocks back to king. And king has like 180hp, one fully charged ring can return 40hp of damage so with 5 rings and 250hp worth food you can let him to kill himself by recoil effect. Note that recoil effect does not count as real damage inflicted by player and that to win battle (to get progress in quest) you need to inflict at least 1 hp of real damage. However, fight will be long enough to get like 5-6 critical hits even with cheapest and worst projectiles or spels you can use. Eventually you can use enchanted bolts, their spells counts as real damage too and opal bolts are cheap enough to fire plenty of them.

Equipment:
5-6 rings of recoil - depending on your activity, stats and luck during fight you need 4-5 full rings to kill troll king
yak armor - it helps agaist grunts in case you have not melee prayer protection. It will help you also on king in case you want to hit him for real damage with melee.
fremmy round shield - most important thing! It deflects most of ranged damage from female trolls and king. It also makes ring of recoil ultimately effective.
one handed weapon - important, because coward needs fremmy round shield all the time. You can take two weapons, one for killing 10 trolls part and one to damage king.
food - you need like 250hp worth food to kill king and about 100hp worth food for trolls killing and to heal on way to cave. There is one honour guard near entrance who can supply you with free additional food (tunas), prayer restore pot and strenght pot.
prayer restore - optional, even if you are prayer user, the prayer restore got from honour guard will be all you need.
energy potion - optional, but for nonprayer users it is recommended to be sure you can run whole way from Neitiznot to cave.
rangers can take accumulator device to save projectiles
if you want to use magic or range bring like 200 cheap projectiles or runes for 200 cheap spells, king has really high defences agaist bioth magic and range. for 10 trolls killing take better ammo or spells. but for king it can be chapest ones becaues all you ned to do is 1hp damage and then kill him by recoils.

Procedure:
First you need to get to cave without big harm. Be sure you answered YES when burgher asked if you are ready for fight. Bring one superergy potion or 1-2 energy potions and wait for full energy before you cross first bridge. Turn off autoretaliate. Go to eastern bridge and cross it. Wait till there be no grunt near next bridge and run over. If you have prayer for range protect, use it. On other side of bridge wait for full energy and then walk to north until you see trolls. Run over them to north switching pray between melee and range protect according to your distance of them. If you are prayerless, you need to heal on way and eventually try to find quest partner(s) to help you to get over this part. When you see north edge of isle on minimap, turn to east and run to most eastern end of it. There should be no trolls around this cave entrance.

In cave you are supposed to kill 10 trolls. Choose female trolls already attacked by honour guards and kill them from safe distance - stay on edge of cave entrance area. Use either crosbow or fire spells. If trolls are attacked by guards, they usually dont fight back so you can kill them easily without taking any damage. After you kill 10 trolls, ask supply guard for spare pray potion (count it as valuable reward) and eventually some spare food. Heal to full hp and run clockwise around central battle area (to not get too close to many trolls) to bridge on opposite side to entrance. If you have prayer, you can switch between melee and range according to situation but rather turn off mele protect before you step on this bridge. King REALLY dont like melee protection!

when you get over bridge, turn off all prayer protection and wear ring of recoil. Step 1-2 squares left of bridge and attack king by cheap distance attacks until you deliver at least 1 hp of real damage. Then you can just stop attacking to save resources and let king to kill itself by your recoils. After king die take his head and teleport away. Do not risk walking back through very angry trolls!