To begin with, I use the build 3/10/10/10 in tdm and 4/9/10/10 in ffa. This is because I think having slightly more regen can benefit you in ffa, due to the fact that you will lack the protection of a base and you are constantly exposed to enemy bullets. Some people prefer to use even more regen, so if you find that that works for you then do that by all means. I just personally prefer having slightly extra health, especially when I encounter other smashers, and I think that 4 regen is sufficient.
When you are first starting out with spike, you may be inclined to take your large amount of health for granted and go recklessly running in to any bullet spammer. However, you should really try to avoid doing this as death will almost be inevitable, or at least you won't get the kill. I would advise you to start practising in tdm with a team mate who is also a smasher. By doing this, you will be able to work together to flank members of the opposite team - whilst they are shooting at one of you, the other can go in for the kill. This is a good method to get used to using spike, because it teaches you that it is largely about distracting the enemy. You will also be able to retreat to base if you lose a large amount of health, so it is ideal for testing tactics out. However, when playing in FFA you will not have a team mate who you can rely on to distract the enemy; you will be forced to kill your opponents independently. This may seem a challenge, as spike is not the best tank for 1v1s against the majority of other tanks. So, in order to succeed in FFA you will have to rely on three main things: surprise, dodging, and knowing when to retreat.
The element of surprise is extremely important, because if you roughly know a player's location then you can do a vertical ambush on them. You can usually figure out approximately where a player is due to their bullet path or the leader arrow. Once you have acquired this information, aim to approach them from above or below. They have a shorter vertical FOV, thus giving them less time to react. This works especially well if they are already using their bullet recoil to move towards you, as they will be unlikely to be able to turn around with enough time to prevent their death.
Another method to surprise your opponents is by attacking them when they are at their most vulnerable, like when they are preoccupied in a fight with someone else. This can work especially well on drone classes, because if their drones are fairly far from them then they will be completely defenseless as you ram them. It does also work on focused bullet spammers, as the bullet spammer will not be able to focus its bullets on both you and the player that it was previously fighting, which reduces their chances of survival.
Despite the spike's large amount of health, being able to dodge is still important for survival. It's fine to take a few bullets, but you should never run directly towards someone through their bullet path. The recoil and loss of health will prevent you from being able to get close enough to make the kill, rendering you either vulnerable or dead. If you don't use the surprise tactics described above, it might be more difficult to win against some tanks, but you still do have a chance. You can dodge around their bullets and drones and be unpredictable with your movements, as this will likely throw them off, and you might be able to get the kill. I have used this on drone classes before, who let their drones get too far away from them in an attempt to get me, as it left them vulnerable.
That being said, you must also know when to retreat. A lot of the time, a smasher's overconfidence can be their downfall, so you should prevent that from being the case. If you have very little health left and you are still charging at someone but not that close to killing them, just stop and try again later. You should be able to retreat from the majority of situations without much difficulty, as long as you do not leave it too late. If you are too low on health to be able to retreat successfully, then just try to ram the opponent as there is always a chance that it will work. However, you should stop yourself from being in this situation in the first place by retreating if you are running low on health and have little chance of success. This is the safer option, and it might require a lot of patience. Spike is not the best tank for great score:time ratios, so it might take some getting used to for a lot of people who main different classes. However, the patience will make you much more likely to succeed, even if it does take a long time to get the kills.
One of the main tanks that will cause an issue for spikes is a bullet build booster/fighter. This is because they are faster than you, so it is not too hard for them to chase you down and kill you. Try your best to avoid them, as the good ones will have little trouble taking you down. However, if you do end up being chased by one, then you can try to make them get away by running through crossfire from other players. They have very little health in comparison to you, so they might be unlikely to follow you. Another more risky thing to do is to suddenly turn around on them when they are chasing you. They will have little time to react so the ambush will hopefully succeed, but if it does not succeed then your chances of survival are little.
Any other rammers can also be annoying for spikes. Spikes are the slowest rammer, which gives them a disadvantage despite their large amount of health. It means that if you have lost a lot of health, you are very vulnerable to any nearby ramming boosters or annihilators. Ramming boosters are my main cause of death when using spike in ffa, because they are very fast and can easily ram you when you have just killed someone so your health is low. If you do find yourself being chased by another rammer, all you can really do is dodge and hope that your health regenerates quickly. Try to avoid making contact with any bullets or shapes, as this will slow down your regeneration time. When you have enough health back, you will sometimes be able to surprise ram them if they are not paying enough attention. When I use the build specified above, most other smashers aren't a problem because they usually put more in regen than me. However, in order to check you can lightly tap them and see how much health you both lose. If they lose more, it will be safe for you to ram them once your health has regenerated. Trappers can also be a pain because they are difficult to kill, but luckily they are not so common. Don't make the mistake of running right into all of their traps. I would advise you to just leave them be.