Just a few comments from my experience on how to make a soloist Fire/Radiation:
1) Take Ring of Fire early! It's your best damage/endurance attack until you get Fire Imps. Slot it 1acc/1endredux/3dmg and you'll have a solid attack that will bring down fliers and stack quickly to take down single foes, not to mention it has above average accuracy.
2) Restrain yourself from taking powers with high endurance cost before you get Stamina (which should if at all possible be done at 20). More specifically, don't take anything from your primary except Char, Ring of Fire, or Smoke (and I don't recommend Smoke, except as a "just for fun" power) before you have Stamina and SOs. As for your secondary, skip Enervating Field until SOs, as well. Before Stamina, the drain on your endurance is just not worth the benefit it provides. Your endurance bar will thank you! Further reasoning: Before SOs, all other powers have accuracy penalties or are too endurance hungry to be useful until you can slot SOs to overcome those drawbacks.
3) Have some form of stealth: Controllers benefit a lot (especially early on) from being able to determine when a fight starts, and where. Keeping certain mobs immobilized far enough away will completely remove them from combat, effectively acting like a hold, and starting a battle from maximum range will give you the best chance of setting things up properly. I prefer Super Speed as my stealth power, because it offers a great travel power, as well (but I do recommend any other travel power as a backup, with CJ/SJ/Acrobatics being my first choice and Air Superiority/Fly a close second. If RoF did not have a -fly component, I would probably reverse the two, but since it does, I opt for the added KB protection, which will be VERY useful once you hit the 20s and Freakshow start showing up in missions.
4) Remember your primary goal is damage, control is secondary. Personally, I feel this is true of any controller, but especially true of the soloing controller. Now this goal includes setting up containment, to maximize damage, so really control and damage are synonymous, but it also means slotting RoF for pure damage, and perhaps slotting Char for a bit of damage (but set up your control aspect first, with SOs, then use remaining slots for damage. Char has many useful configurations, so figure out just how much hold you want, in addition to slotting for endurance and recharge to taste, and if any slots are left over, throw damage in there).
5) Have Accelerate Metabolism and Hasten by level 12: These powers, when alternated, will give you a boost to your damage and control that will probably save your life on more than one occasion. Only use them simultaneously in emergencies, otherwise, use one, then the other, alternating. This way, you will almost always have one running, which will speed up your arresting rate, and enhance your surviveability greatly.
6) Learn your mobs: If you are new to the game, pay attention to the range and status effects of various mob powers. This way, you'll learn which mobs need to be held immediately, and which should be immobilized at range to completely negate their powers (Buckshots and Flamethrower mobs, for instance). When you get really good at this, you'll find that even longer range mobs have a bit less range than you do on their attacks, in some cases, allowing you to negate many mobs just by ensuring you immobilize them at the maximum range.
